Where can you buy a butt plate for a 1912 model 14A 35 Remington slide action pump?

Watch e-bay (they allow parts like this) and gunbroker.com

You can also try brownells.com. This is a gunsmith supply house that has vintage replica butt-plates for Remington rifles. These are made just like the originals but will need to be fit to your particular stock. This is no biggy, you just need to blend the sides of the oversize plate with the contour of the stock. No two stocks are exactly the same so if you get one off ebay, in all probability it will be too small or too big is some areas around the outside edge.