Again, you should be able to find one or two brands in most better health food stores. Then again, I live in California, it might not be as easy in other places. They'll cost around 5 to 10 dollars each, which seems like a lot, but remember, you use them over and over. Try one or two out, and if you like the concept, buy several sizes and shapes to last you through your period.
If you can't find them in the stores, here are some mail order sources: