There are no legal download sites for Riddle of the Sphinx.
You can buy a disc version from the allvideo guy on eBay or on his website. That's not a downloaded version though, it's the original game on CD plus an installer disc that runs the game in tweaked DOSBox on modern Windows.
I think Big Fish should help you with your problem accessing your games though -- unless it's a problem with the game not being compatible with your Windows version and not to do with the Games Manager.