Darn near impossible to get one from WW2, for example. I got one of the last 1945's three years ago. I'm sure they're around, but not for the $500 I paid.
Depends on the grade. The WW2 guns in good enough condition to sell are pretty well gone- seems like the service grade guns tend to be later production. There are, however, WW2 serial numbers guns available, but they are reconditioned to where there is very little collector value.
The CMP Special grade I received recently has a receiver serial number that puts it into December 1942 production and it does have some WW2 hardware (it isn't fully WW2 correct, not with the new CMP stock and Criterion barrel), but it's all been refinished so it's in basically new condition.