supergrit and trugrit are good places to buy. Pops might have some as well. I've got a huge collection of 1x30 belts, mostly from supergrit. Depending on what you want to do with them, there's other types of belts besides the AO or SC belts that you can use (cork polishing belt, for instance).
If I might suggest--fill out your profile. Some of the makers in your area might be able to help you out.