Buy one that is already on a bottle, it is not a good idea to buy one that has been on it's mom and put it on a bottle, especially if it's over a week old (and you are inexperienced).
Bottle babies are a big committment, and very time consuming. Depending on the kid's age you will probably be feeding 3-5 bottles aday, although you do not need to feed them through the night (sometimes you do w/ a weak newborn, but I would assume you'd get one past that). Up to 2 weeks I generally feed 5 times aday, then depending on the kid if they are ready I push them back to 4 times, and keep them there until 8 weeks. Then 3 times until 12 weeks, then start weaning. Although alot of people seem to get bottle burn out after about 6 weeks, so I recommend getting one that is 4-6 weeks old already.
I'm feeding 4 right now, the oldest three will be here til they are weaned, they younger one is going to his new home at 6 weeks.