Ebay might be your only choice. They can also be difficult to get running on modern machines, so you might have to wait until Bethesda releases them for free with a patch included.
I agree that Ebay's thew best bet, but sticking something on Craig's list may help. Still have my copy. But haven't tried to run it in ages. Got an old XP machine upstairs. Might have to give it a go.
amazon or ebay, check other countries stocks aswell. I Bough a mint condition unopened original release of Redguard for £30 but used copies can be had for cheap. Dont pay stupid prices for it, $20 or less.