Right now you can't get a copy of System Shock 2 legally. It's pretty much impossible. Even if you could, it'd take so much effort to actually get it to work that you'd probably regret the choice.
The IP rights are such a tangled mess that it's prevented anyone from releasing the game in a digestible format for years. System Shock 2 is one of the best games ever made, but it's plagued with not being developed with forward compatibility in mind. It won't run on newer machines without a significant toil.
Thankfully this ends tomorrow, with Good Old Games having worked out the licensing rights for digital distribution and selling it for $9.99. In an interview with Rock, Paper, Shotgun they spoke to Stephen Kick of Night Dive and Guillaume Rambourg of GOG who've been able to properly allow players to rediscover the atmospheric space-horror over again.
Guillaume tells us the team at GOG had been working on getting System Shock 2 onto the service for essentially as long as the site has existed, speaking to an assorted cabal of lawyers but not really getting anywhere. They'd almost given up hope until Stephen Kick managed to figure it all out, secure the rights and approached them. GOG have since adjusted the game to iron out a few of the infamous bugs that previously the fan community had to try and work out with mods, but it mostly remains largely unchanged.
If you haven't played the game before, really, you should absolutely do that when it hits early tomorrow morning.
