Enameled “Magnet wire” is available in all sorts of gages, for example this assortment from Radio Shack
I’ve never seen solid wire inside a computer cable (no, not even the double-density IDE and SCSI cables), but as people have pointed out, 30g wire-wrap wire is still pretty commonly available. A good surplus store will probably have some 26g wire-wrap wire as well. (hmm. DIgikey seems to have quite a variety of WW wire: 24, 26, 28, 30g. Not cheap, though! This does not seem to be a good thing to try to buy from digikey!)
Telco cables (including cat5 ethrnet) tend to be 24 to 26g, if not stranded.
Bare wire is available in craft stores in assorted sizes. I’m not sure if there is anything down at the 28g+ level (Hmm. You can get niobium 28g wire for $0.28/foot, and titanium 30g for about $0.07/foot from http://www.reactivemetals.com. Anodizes to pretty colors…)