If you have a Fry's electronics in your area, go get some of their house brand 12-ga. I think it's GC, GQ or something like that. It's price isn't all that great though, but if you need it now, they can do.
Also, if you manage to find a radioshack that still sells 12ga, go for it! Their 12ga is really good stuff....but they discontinued it a while back.
Another good source for 12ga is car audio shops...if you find a nice car audio shop, they won't charge much for their bulk cable at all. I've seen 12ga as cheap as 50c/ft when 30ft or more is bought at some of these shops.