Something cheap, XP or Vista trying to run this on bootcamp I know I posted something like this earlier but, there are more players here now.
In a store. Other than that look at newegg.com and check the system builder versions, different licensing but more affordable.
I thought once a new windows comes out that the older ones are usually sold and pulled by microsoft, saying that I think win7 is now around 79 if you can afford it. As for finding a copy of xp you might be out of luck, it's hard enough to find vista anywhere now.
Strangely enough, even though XP is outdated by 2 versions, it is still quite pricey. At least $75. Compared to a brand new install of Win7 $100. So to you, I suggest perhaps Windows XP Black Edition, if it's just for running games. I'm pretty sure Windows keeps the price the same so it's entire audience isn't on the previous OS because of price drops after new ones. :evil:
Are you going to school? College students can get buy Microsoft products for dirt cheap if they them through their school, something like $30 for a copy of Windows.