You're going to have a rough time finding them, but you must have them. Do you want the first ever originals or dont you mind having the thx digitaly mastered version (its the originals but with better sound)?I'm desperately trying to find the Special Edition of Empire strike Back(gold box edition).
http://www.amazon.co.uk/exec/obidos/search-handle-form/026-1005069-3397256
Searching around i came across the original (well 1997 re-release) of the original trilogy boxset, this was the gold box set with Darth Vader cover, above is a link to the page where the above information can be found.
As for VenganceGods enquiry there is also single edtions of A New Hope and Return of the Jedi below are the respective links:
http://www.amazon.co.uk/exec/obidos/search-handle-form/026-1005069-3397256
Hope that is of some help, good luck in your search. 🙂