You can get a good deal on ebay, just don't expect them to work. The way I take care of things, I just work on whatever OS was on the PC when I work on it, but that is used stuff of course. I get some old P4 boxes with OEM stickers, I install XP, no cost to me, they get a valid licese, no laws broken.
You will not get a large discount unless you buy in the thousands, even site licenses are not that heavily discounted. For someone like an HP or Dell, they can get prices that are about 1/3rd of OEM retail. Although my info is from the XP days. Maybe Vista and 7 are more strict on prices.