There really isn't any other place outside of Ebay. Well, maybe Half.com but thats run by Ebay. Have you checked Froogle (Google's Shopping Search Engine) or Yahoo! auctions?
About three years ago, I had to buy mine out of the UK in a EA set (with Alpha Centauri and C&C Red Alert) for about $30 after shipping. You might have better luck doing that - though I haven't seen that particular set in circulation in a long, long time.
Yeah, ebay is your most likely bet. The other option is to check around via google for some websites that sell older games. There's some out there although I don't know their URL's right off. Hopefully you can locate a copy at a decent price.