They aren't going to buy REAL helmets. Nazi or otherwise.. It could cost a fortune.
A shop that I worked at was hired to provide helmets in the past, and after some scouting a local prop house had a surplus of reproduction Nazi helmets. We ended up making a custom lid and producing them for the film, but if they didn't have the budget they'd have had repurposed Nazi buckets as well.
As I said, I'm sure the costumers went out and found the easiest and cheapest helmets they could in bulk, and they were reproduction Nazi helmets.
I know someone who worked on the costumes for the movie and I'll ask what his take on it is.
Maybe I wasn't too clear:thirty years ago the most common prop helmet would be ether German,American,or Brit from WWII but not nowadays we just don't have that many WWII films being made to have a lot of Nazi helmets laying about,however the Kevlar ones are teh standard and are everywhere and I've seen ABS ones for like $25
Go ahead and ask his answer might be interesting,my point is somebody had to go looking for those things I doubt you could just find them laying around before you'd find about a million Kevlar lids of all descriptions first,anything from costume jobs to actual surplus for cheap.