The parts aren't as difficult to find as you might think. The seat cushion from 944S2 cabs will obviously work, but so will the lower seat portion from any 944 with the split back upper seats. The seat I got was from a 944 coupe that had the split upper seat backs and it fits perfectly.
Jay's point about reupholstery is correct and dependent on what year 968 you get and what color your interior design is. The front seats for the 1992 through the early 1994 968s match the 944s so the back seat patterns will match correctly if you can get the same color. For the late 1994 and all 1995 968s the front seats match the 993s and have a different upholstery pattern, and the 968 coupes rear seats only had the upper folding backs plus two individual little cushions for the butt portion of the seat bottoms. Perhaps you could make the coupe butt pad cushions work but I've never seen nor heard of anyone doing that. Another alternative for the later cars is to use the early style rear seat bottoms. They aren't a perfect match but unless someone is knowledgeable or really anal most people won't even notice as long as they're the same color.
For my cab I got a cobalt blue 944s2 rear seat even though my car has the midnight blue interior. It's not a perfect match but you have to have the right light for a casual viewer to even notice. Having the back seat in there has really helped on several occasions when I've has both of my daughters in the car, and a couple of times I've even had one of their friends along for a full complement of 4 occupants.
Definitely don't be too worried about this modification.
