Check out a local fabric store, or even Wal-Mart. The pattern books have them listed under uniforms & scrubs. Since I am not that great of a seamstress, I had my mom's friend teach me how to make them.
The pattern I used was made by Simplicity & it was simple. Just cut out a big circle & you've got it. If you have a lot of hair like I do, make your circle bigger than the pattern, or else your hair will never fit. I also found that using 1/4 in elastic didn't leave that awful mark on your forehead.
Those elasticized circles are ugly, I hate them. Plus, elastic breaks my hair. There is a woman out west who makes the most gorgeous baker's caps for us, carewear.com. I get so many compliments on my hats and have over 20 of them in many colors and patterns. She charges $12 per hat and she pays the postage. Well worth the money, I guarantee it.
AORN won't support use of these caps unless you can prove that there is no increase in infection rate. They are concerned that you can't monitor if people are washing them ... Hate to be a bummer, but JCAHO creamed us on this one.