You have 2 choices... you can get the PC version (for dirt cheap btw, less than $10) and run it under bootcamp, which I would recommend as it's a newer update... there are more servers to pick from... and the texture mapping is far superior to the Mac version.
If you buy the MacSoft Universal Binary upgrade for $5 you'll need a Mac serial, which is going to cost you. You'll pretty much only find them on eBay, and they are at a premium since they are rare, so expect to pay in the $40-60 range, I got mine off eBay for about $55.
You can buy a Universal Binary complete install disk off there too, but probably will run a little more. But Gamestop is probably you're only other bet (and probably a cheaper option than eBay as they may not realize it's current value). It's not going to show up on a major retailer, like Amazon or the Apple Store, it's out of date and long off their radar as a product to sell seeing it's out of print.