Seat covers

Good morning. I have a black 1964 Valiant Signet convertible with black interior and I live in the central valley of California. The problen I have is the seats get extremely hot in the summer. I have a set of old and worn out sheepskin seat covers. I would like to replace them with something new but can't seem to find any. Does any one else have the "hot" seat problem and if so what are you using?