I found a nice original 1928 Montana plate on eBay. The cost depends on the condition, and how rare your year and state is. Mine cost around $75 IIRC.
As far as being legal, it isn't....but, you're unlikely to have any problems, as long as the proper issued plate is also displayed. This doesn't mean you won't have a problem with an individual policeman who will make an issue out of it, but the majority of cops will understand why you have a vintage plate on your vintage car, and will let it slide.
Make sure you have the tools to take it off, on the spot.
Be nice, don't be a jerk.
(Put it back on later.)
eog