What you are looking for are generically called "jewelers" screwdrivers. They come in a set, usually of 6, 3 flat blade and three phillips. Cheaper sets tend to have one body with interchangable tips, better sets have 6 individual screwdrivers. If you google "jewelers screwdrivers", you'll get several pictures of what you are looking for. I put a 512 in my iBook today, and that's what I used.
Available at any big box hardware store. Believe Sears sells a craftsman set as well. A set of very small allen wrenches is also handy, say for field stripping an apple keyboard so that a fella can get root beer out of it. Hypothetically, you understand.