You can get a T-rex a lot of places, go to your local cycle shop and ask them. Parts Direct or one of the big parts dealers that supply most shops carry them. I am not sure what catalog I was looking at the last time I was at the dealership but I saw it .
To answer why drag racers use these cylinders, they are the best. They are a far supeior design than stock cylinders. The answer to that lies in the transfer ports.