Corp Kit Agency is/was fantastic. Dunno if it still exists in the same form though. I rented No.1s from the place that now does it down in Rowcroft Barracks (old Depot REME). Phone no. is 01189 763 452.
Depends how much of a rush you are in (and what size you are). Doing a search for REME on Ebay usually brings up 2 or 3 sets.
i need a Duke of Wellington mess dress, anyone know anyone who may have one to sell? Johnomissus, my bro in law is in REME, i can ask him for you if you like, he may know someone who is getting rid of theirs.