Probably nowhere. Microsoft is likely going to reserve the download rights for themselves only.
Also, I'll just say that you should be very wary of buying anything from TigerDirect. They have a less than stellar reputation, and their parent company is known for doing some shady, borderline legal, things.
Besides the Microsoft Store (online), does anybody knows where can I buy Windows 7 as a digital download?. I've tried Amazon, BestBuy, TigerDirect and they don't seem to be offering it as such...