I am aware of various sources to get "fake" plates. Actually the plate stamping places in Germany now make you any plate configuration you want. I got a few for friends of mine for their German cars, showing personal letters and numbers.
This "Mainz" plate, however, is authentic. It is not a custom made plate and was issued to an original Mainz district car.
In addition, it is from the late 70s,early 80s before they put the blue field with the "D" on the German plates.
Maybe you should travel to Germany and source something authentic at a junkyard? Or, maybe scour the dark web, or eBay.de for same? Surely there is some market for old plates in Germany, even if not precisely legal. There are a few vintage authentic German plates for sale now on eBay in the USA. None apparently from Mainz, but keep looking. Maybe on eBay.de?
If you look carefully at that plate in the photo, and then again, at that
one vendor's selections, you can see that they'll make that plate for you, yes, without the blue field and D. They have old, new, registration labels, etc. Is it authentic? No. Is it authentic looking? Most would say yes. I had some Germans looking very carefully at my plate and commenting on how exacting it was made. I just wanted mine to answer the two questions I always received about the car when taking it anywhere: what year is it, and what model is it? So my ersatz German plate says 1969 280SL.