Well, the good news is that padding or re-padding the rear seats of a 1st gen Camaro with new foam is relatively simple and not very expensive. The seat bottom and seat back are basic bench seats with not many frills, they are very straightforward. That's why there is no foam available for them.
Look at this thread: CLICK HERE
This is a '53 Chevy rear seat, but the process is still the same in your '68 Camaro.