Typically from one of two online stores:
I look for my discs first at communitydiscs.com, as the shipping is one dollar and the guy who runs it really cares about his customers. However, he doesn't have the widest selection, so I also buy discs from Marshallstreet.com, huge selection and documents every disc.
I also will pick up the occasional disc from Academy, or at a pro shop (I have a lot of discs from Killer B). There are also a few other websites I've purchased from, but the two I've listed are the ones I prefer to support.