It's awesome that you're want to foster, but if you're looking at a fair amount of work if you're looking specifically for kittens that need to be bottle-fed. I have never personally fostered as I am generally maxed out on animal capacity with my own pets (I have 4 cats and a horse right now!), but I do follow several rescue/animal welfare organizations and have bottle-raised a litter of puppies, so I can give you a bit of info.
The easy part is finding a rescue with kittens in need of fostering. Try googling local rescues or contacting large animal welfare groups, such as the American Humane Society, who may have contact with smaller local rescues. Another good website is petfinder.org. This website lists animals up for adoption, but they include the names and contact info of the group offering the animals for adoption. Chain pet stores, such as PetsMart and Petco, are also often associated with local rescues and may even have a list with phone numbers and addresses of local rescue groups. The local pound is a great place too. City- and county-run shelters do not have the resources to care for kittens and puppies who need to be bottle-fed, so any turned into them must be put down if a foster home isn't available (which one usually isn't). If you leave them your name and number, they're likely to give you a call when a litter comes in. The hardest part right now about finding a litter to foster is it actually isn't kitten season right now (yes, there is such a thing), as most animals prefer to give birth in the spring, when the weather is milder and resources are more plentiful. Not that they're not out there, there just aren't as many.
Once you find an organization, you must determine whether this is the right organization for you to foster for. Some rescue groups have the ability to pay for all expenses for you while you foster, but most require that you pay some or all of the expenses for the animals' care. Believe it or not, kitten care can become quite expensive. If the kittens were pulled from their mother during the first 24 hours (or abandoned), their immune systems will be weak to non-existent as kittens (like people) must receive antibodies from their mother through colostrum. There is some commercially available colostrum-replacement, but it's not as good as the real thing, and the ability of the kitten's system to absorb and utilize colostrum drops dramatically after the first 24 hours of life. As a reference, the newest member of our family cost us somewhere between $300-$400 in vet bills in the first 4 months as he repeatedly spiked high (life-threatening) fevers. This amount doesn't include shots, etc., and we found him in the alley behind our house, already very sick and very underweight (but, fortunately, not in need of a bottle). Also, does the rescue handle abuse/neglect cases? If so, will you be asked to foster any kittens who are part of a legal case? Could you be asked to return them to a previous owner, and if so, how will that be handled? Could you handle giving the kittens back to a person who may not give them what you consider ideal care?
And now, to the hard part - the actual care. Kittens are essential totally helpless for the first 2 weeks of their life. They are blind and deaf and need to be fed approximately every 2 hours (the time factor is what makes it impossible for most shelters to care for them). They are also incapable of controlling their own body temperature, so they must be kept warm. A nest made of a blanket or 2 in a room in the mid-70s is usually good, and they often like to snuggle in a little blanket cave (cats have higher body temps than humans, which is why they like snuggling as adults too. It's also why they like laying on your TV or computer
).
Making a bottle for a kitten is a lot like making a bottle for a human - follow the instructions on the can, heat it up, and test it on the inside of your wrist or forearm, whichever spot is more sensitive for you. If it burns or feels warm for even an instant, it's too hot. It should basically feel totally lukewarm, almost like there's not even a drop of formula on your arm. I prefer powdered formula b/c it's cheaper and generally can be stored for a little longer than liquid formula. Shake the bottle well, and be ready for anyone not being fed who hasn't eaten yet to cry and cry until they get fed. Let them eat til they're full; kittens don't often have a problem with overeating and spitting up. If they do (formula can, occasionally, be a little heavier than actual milk), shoot for smaller, more frequent meals. Once they're eyes and ears are open and they are starting to show a little more coordination (anywhere from 2-4 weeks, depending on the kittens), start offering formula in a shallow dish, maybe even a plate. This is the beginning of them learning to eat on their own, but they must be supervised (drowning risk) and expect to still need to bottle-feed. Different kittens learn how to drink on their own at different times; don't rush anyone. Once they have that down, start offering kitten food along with formula (anywhere from 4-6 weeks). At first, they won't be able to eat dry kitten food, so you'll need to offer either canned kitten food or make a mash with dry kitten food and hot water (the temp rule still applies when you offer it to them, same as formula). From there they can be slowly introduced to regular kitten food. In emergencies kittens can be weaned as early as 6 weeks, but it's better to plan on no earlier than 8 weeks. Some kittens even benefit from formula past this age (such as if they start losing weight after being switched to kitten food.) As for growth during this time, the rule of thumb I found is 1 lb for every 4 weeks, so at 4 weeks they should weigh around 1 lb, at 8 weeks about 2 lbs, etc.
Beyond feeding, there is the other end of the digestive system. Very young kittens are incapable of pooping and urinating on their own. In normal circumstances, mom helps them with this by licking their genitals and anuses. Without mom around, it's your job (but you don't have to use your tongue!). A short time after feeding (maybe 15 min, but experiment a little and you'll probably find the best time), you'll have to go through and wipe each kitten. I have heard of people using q-tips and cotton swabs, but I prefer a (dedicated) washcloth. The washcloth is more sturdy and slightly rough, more like mom's tongue. Whatever you choose to use, soak it in warm water then ring it out to just not dripping. You may have to re-wet between kittens, depending on what you use and how many kittens you have. Not to sound dramatic, if you skip this step, the kittens can die of hernia (we lost 2 puppies this way in a litter we rescued. The woman who had them had apparently taken them away from mom sooner than she admitted, as they had already herniated by the time we took them home.) At about 3 weeks they are ready to be introduced to a litter box. At this point, they are best kept in a room or fenced in area on a hard floor, such as tile. The only soft spots in this area should be their bed/blanket nest and their litter box. You might even consider putting a box over their bed, making it a safe hiding place too. This makes the litter box the most desirable place to urinate and defecate, since cats don't like having their legs splashed. At this age, most vets recommend not using a clumping litter as it can cling sensitive areas, especially if the kittens lose their balance and fall in the litter box (which they're likely to do). I found the cheapest and easiest alternative is shredded newspaper. You can either shred by hand into strips or just run through a paper shredder.
At about 6-8 weeks of age they are ready for their first shots, although you should consult with your (or the rescue's) vet, as some vets have slightly different time schedules (shelter vets are sometimes willing to vaccinate and spay/neuter at a younger age as it is so important to find them permanent homes). Remember that if any of the kittens are showing signs of illness (especially fever), vaccinations should be delayed until everyone is healthy. Until they can be vaccinated, none of the kittens should be exposed to any other animals that are not current on their vaccinations, including family pets.
Other than that, have fun! Keep lots of toys for when they get old enough to play. Your vet is the best resource if you have any questions once you have the kittens, and a good vet (or vet tech) will be willing to talk you through just about anything on the phone or in person. And if they don't get adopted out right at 8 weeks, you might get to watch them lose their baby teeth (at about 12 weeks). Don't be surprised if you don't see them, as they usually swallow them (I think I had helped raise at least 10 kittens before I found my first lost kitten tooth), but our 3 youngest all sported double canines for a couple days when the adult tooth grew in before the baby tooth fell out. And once they're all adopted out to their forever homes, congratulate yourself for making the world more playful and beautiful. Good luck!