Virtually any place you register a .com you can buy a .org. Eg. godaddy, yahoo, the one above. You're probably going to want to shop around some to find the best prices. I would personally recommend that you stay away from yahoo because I've found, in the past, that they throw on a lot of little charges that really do add up. I have godaddy right now and it seems reasonable for me but I recommend you look around.
Also, and I highly suggest this for any domain name registered, you make sure you have your domain registration privatized. I didn't for the first few months and realized that was a really big mistake. It should be an option when you go through the domain registration process, if there is no option to make the registration information private I would highly suggest you see about buying a domain from a different company.