Brey Krause makes a number of seat-to-slider or seat-to-floor adapters. Here is what they have listed for 964s:
http://www.bkauto.com/category_s/215.htmAlthough I use their seat brackets that adapt to the factory sliders, I eliminated the sliders (and gained some helmet clearance) by replacing the sliders with some custom flat steel bars that connect the side mounts to the factory slider mounting holes in the floor.
Some of the local shops can get Brey Krause, but I've also used
http://www.OGRacing.com and
http://www.LPIRacing.com (no affiliation, just a satisfied customer). The two online shops also carry the "Planted" seat bases for 74-98 Porsches (which actually look like a pretty good starting point, from the pictures), if you don't want to mess with floor adapter fabrication, but I don't have any experience with these.
One small detail to pay attention to is if you want to retain the factory seat belt latches (say, for street use), you will need to make sure you have a threaded mounting point somewhere on the seat bracket or the seat base to attach them. If neither comes with one, Brey Krause also makes a "bolt-on bracket" to add it.