I think www.mcmelectronics.com is running a special on flat speaker wire this week/month. I got a new catalog in the mail and remember seeing something about that in there.
If price isn't much of an issue, you might want to check out http://www.decorp.com
They have the ultimate in a flat wire. It's rated to 14AWG or 18AWG. It's about the thickness of a business card. Never tried it, just found the website.