There's a prop warehouse just off the M25 around London and the last time I was there, they had a full sized X-Wing fighter and a few other props from Star Wars. I was told that while many are replicas, a fair number were in fact original, but I'm no expert and couldn't tell.
Other original items they had were The Mystery Machine van from the live action Scooby Doo, a couple of armoured sci-fi type vehicles from a movie I should know**, but don't, two ships from Space Above and Beyond and one of the motor bikes from Judge Dredd. They also had a boat load of smaller props, weapons and gadgets from all manner of movies such as Bond and so on, all piled up on row after row after row of shelving units.
The best thing about all of this is that you can hire the stuff if you want, which would be good for taking measurements and reference photographs if needed.
Holluba
**Having just thought about it, I do know that one of these vehicles was used in the movie, Children of Men, but it was built for a much earlier production, possibly from the very late 1990's to early 2000's.