Where to buy Windows 7 for Parallels?

I'd like to run a few Windows programs on my new MBP with 8GB of RAM.

I'm thinking Win 7 Home Premium 64-bit. What's the cheapest way to get a legit copy? I see all kinds of offerings on eBay, but I'm not sure which will work and which won't. Some say OEM is the best way to go, yet others say it's really not legal.

I would appreciate any suggestions on where to go to do this inexpensively but legally. Thanks.