In movies about World War 2 for example, what kind of tanks are used to stand-in for German Panzers? Are they just modified British or American tanks or are they actual surplus Panzers? Is it really of much concern as most people probably wouldn’t know the difference anyway?
AFAIK, there aren’t any “surplus panzers.”
As for substitution, it depends on the film-maker and the budget. For The Battle of the Bulge, they used US Patton tanks painted in Axis colors. Today, with most film-makers at least aware that there some of us who can tell the difference, they seem to be trying harder. A little fiberglass, a little sheet metal, and you can fake a PkWIII pretty closely. 
Not realting to tanks, but as far as equipment stand-ins in movies, I briefly worked as an extra in LA on some tv shows and films, and once my car almost made as much as I did for the day, simply by letting them park it on the “New York Street” at Paramount Studios. They muddied up the car, and after the shoot, washed if for me. So I got paid for it to be in the shot, got great free parking for the day, and I got a free car wash.
Hmm…now that I think about it, that means the car actually earned more than I did, all things considered…