if you live in a state where amazon isn't, thats usually the cheapest way to get them, tax free and shipped free if your order is over 25 dollars.
also, once in a while circuit city and best buy will have a promotion where you buy a new release and you get a point card. (gh2 launch at circuit city)
theres also xbox360achiever.com, and some others.. check out this thread. i did that one and it worked great.