Re: Remington 51 .380
I know of no source, but you could try the usual suspects, like Jack First and Numrich,
Making a plate is no problem and you have one to copy, so it is just a mirror image. You can get thin steel plate at some hardware or big box stores. This is one area cold blue will work fine to make it look better. You don't even need to drill the rivet holes if you are gluing the grips on. If you don't have the rivets, just get a couple of nails and round off the heads to the right contour, then cut the shaft to the right size and cut the heads off with 1/8 or so shaft for sticking in the hole in the grip. Glue them in, daub a lttle cold blue on them and it will be hard to tell they are not the original rivets.
(See, I do recommend cold blue, sometimes; just not for whole guns.)