Don't have the URL's for you on the tip of my fingers, but you can do a a web search for
Tripp Research
Accurate Plating and Weaponry
Techplate Engineering
The first two I've read good things about, not many folk have heard of the third
A keyword search "firearms coatings" or "firearms plating" will likely yeld many hits
I'll check for the web addresses of the ones I've mentioned and post them later.
Good luck