Tip,
Several years ago I just google searched "zinc plating Fort Worth Texas" and my place popped up. All they do is plating. They have silver zinc, yellow zinc, black zinc, and several more. They also do mail order work too. I was just talking to him yesterday about that. He says that part of the business is growing.
http://arrowplating.com/processes.html
The minimum charge is 25 dollars. That gets you up to 25 pounds worth done. I've never had it exceed the 25 dollar charge yet.