This time I must disagree with ChainsawAsh.
He is correct if you are looking for SW 2.5 or 2.7, ESB 2.0, or ROTJ 2.5.
But anything earlier than that probably won’t be very easy to find. I wouldn’t worry about it though.
About the only reason you’d want to find earlier versions is for comparison purposes. Each subsequent release is just a more highly polished edition of the films.