Good idea, thanks!RJP:Better than that, they are FREE at bike shops...
I simply ask them for one from the trash bin. Some poor soul just paid $10 to have his tire changed and the shop took the old one with the tiny little hole in it and tossed it.
My local shop now knows to save the really large ones from the fattest mountain bike tires for me.
One tube gives a LOT of rubber bands, so I'm still working on the two spares I had from my long-since stolen (and never replaced) mountain bike. I paid the couple bucks, 6 years ago, because frankly I wanted the new, non-holey variety for my bike. Good tip, tho... when I finally finish going through the second spare tube (I'm still on the first one!) I have I'll pick up more at a bike shop. Now I just have to find a full-size inner tube source...