I simply bought a 68 Mustang car cover and it fits great. I highly doubt they make anything GT/CS specific. There isn't enough difference in the cars to make a difference in the covers. They will work, mine would work perfect on a fastback, convertible etc it wouldn't make any difference. About the only option you need to decide is the quality, it seems the come in about 3 price ranges and each one adds a little something to make it nicer.
Hook