If you're looking specifically for a factory radio from an 86 buick, check out www.car-part.com as they show what's available at junk yards all over the nation (most of which are willing to ship).
If you're just looking for a replacement CD player/stereo, Walmart carries some okay universal ones, as does Best Buy. Otherwise, look around for local car audio specialty shops as they have a better selection and will install the CD players as well.