You can get them at any hardware store pretty cheap. Just take a wheel in with you and see what fits. You'll find several sizes that have the same inside diameter but different thicknesses. Get some of each and see what works best for you.
Here's a link to another thread talking about this very subject back in '07. This discusses what I discovered, now use, and what I do with them. I've also thrown a link in here to the supplier. A pack of 100 of these o-rings is $2.10 before shipping and whatnot.