I understand the difficulty in getting German tanks, but the M4 Sherman was one of the most mass produced tanks on the world with over 50,000 produced from 1941-1945.

By the 60's and 70's most of them were either in museums, became monuments, or were scrapped. The US was in a position where it was dumping them into the ocean because it was cheaper than shipping everything home. By the 1950's everyone had either either converted to Soviet tanks- Eastern Europe primarily- had converted to domestic productions- Britain and France- or was using tanks that the US was producing- not M4 Shermans post-Korean War- but was more than happy to sell to anyone who was allied with them. The civilian market we enjoy for tanks wasn't really there in the 50's and 60's and 70's, and what was left on the 'free market' had been bought up years prior; Israeli armed forces made extensive use of significantly modified Sherman tanks.

In the case of the movies Battle of the Bulge and Patton, what you really want to know is where they were filmed. Though it'd certainly be possible to acquire and make use of period-accurate tanks, that'd be expensive between the skilled labor necessary to maintain them, logistics to keep them fueled, and simple transportation. In this case, the movies- or more specifically the battle scenes- were filmed in Spain. The Spanish army didn't have Shermans but the US had supplied Spain with about 400 M47's and I'd have to go look up how many M48's they'd received.

Spain had trained operators, and could manage the difficulty of keeping the tanks they already had maintained for a cost relatively affordable relative to fielding period accurate tanks.