I know there are quite a few places to buy them which you can probably find on the internet with a google search. Ebay would be a good place to look as well.
If you are interested in making your own for pretty cheap you could do one like this:
This is a stall plaque I made for my mare. I just used a pre-cut blank plaque which you can get at Walmart or Hobby Lobby for about $4 or $5. I spray painted it purple and added 2 vinyl decals (which I make) for the names. It has both her barn name, Lenore, and her registered name, Ten Talents.