Damn Picky... just kiding. That's where I'd usually go. Try Home Depot or Lowes maybe. I've bought a spool of 12awg speaker wire there before for pretty cheap. No idea if they have power wire that big, in a style you would want, but they have a ton of spools of wire (you can buy whatever lenght you want too). Go back to the automotive section in walmart. They have stereo wiring kits for the cheapest I've ever seen anywhere. You could probably get a kit with extra stuff in it for less than what you are talking, but who know if it will be red (probably?). I got a FULL wiring kit (RCA's, power wires, everything) for $25!!! that said it supported up to 1000w (something weird like 5gauge wire, 80 or 100amp fuse) recently when doing a buddies stereo. They jacked the prices up some since then. I know they also had a 'power' wiring kit about that size for pretty cheap.
You can always go to any car stereo store, I'm sure you know this, but you'll pay more than that.