Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Purchasing as a student (if you can) is usually the cheapest
Other than that, you can usually pick up a cheap OEM version from NewEgg I believe
Or perhaps on eBay
 
is the OEM from Newegg all I need? I just need the basic cheapest one for running some games, everything else I'll be doing in OS X
 
ask to borrow your friends windows laptop and when he's not looking take a picture of the sticker that's on the bottom.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.