Saying you need parts for an old GT Performer is kind of like saying you need parts for an old Ford Mustang. They've both been in production for a really long time, and exactly which parts you'll need will depend on exactly how old it is. (Not to mention what's actually broken in the first place.)
The good news, though, is that most parts for your bike should be easy to find, regardless of age. Post more info about your bike's specific problem(s) and we should be able to point you in the right direction. And if you can post pictures, it would be a huge help.