I got mine from Dell. I got the Xbox 360 Elite Holiday Bundle for $70 off and free shipping. I had been watching for a while and seeing all the discount codes, but everytime I finally found one, it would not work. Apparently they put restrictions on them where they can only be used either 1000 times or, like mine, only 400 times.
I got lucky one day by doing a search for "xbox 360 elite bundle" search and found a new code that I had not seen before. I tried it and it worked. As a matter of fact, mine is being delivered today.
Just keep trying!