Am I mistaken or System Shock 1 was sold here and now it no longer is?

That's a shame, I guess I was wrong.

Thanks but I don't care about any portable games.

System Shock Portable is a fan-made port of the original System Shock that comes bundled with DOSBox to get it running easily and also includes mods for high resolutions and mouselook. It basically makes the original game much easier to play today. The "portable" just means you can put it on a USB stick and run it on different machines easily, it's not some sort of port to mobile devices or anything.

Has someone managed to get SSP to run on Linux/Wine? I've run my cd version in dosbox, but I'd like to make use of the improvements (and speed) of SSP.

I did once run SSP on OS X, it's still a DOS game that can be ran in DOSBox. SSP just bundles the Windows version of DOSBox with the game so the player doesn't have to set up anything, just like GoG does with their old DOS games. Simply take anything that isn't DOSBox and run it in DOSBox on your OS of choice.

I think SSP uses the gulikoza build, I think, which affords it more speed than the usual dosbox. At least, when I ran it in dosbox, it was painfully slow at higher resolutions. With SSP, no problem.