Anderson's in Elkhart is definately the best place for replating. I'll add this bit though, make sure the horn has been completely worked before sending it off, set all your posts, pull your tone holes, file them, remove your dents, clean the horn thoroughly. Just like painting a car, the end result is strictly a product of the preparation. It might even be beneficial to do a mock overhaul of the horn once, setting the tone holes and posts and such, and then RE-disassemble the horn and send it to em. They do a beautiful work!
-Pat
-Pat