I've used Delta Specialty Coatings in Houston for about twenty years to cadmium plate steel carburetor components. You can ship to them and they will return ship. While I have probably 98% of my stuff done in silver (clear) cadmium they have also done gold and black zinc on steel for me and alodine on aluminum. They have a $95 minimum so it behooves one to send them a box full. They do not polish things they are industrial platers.
https://www.deltaspecialtycoatings.com/contact
You probably do not know me but while I have the last name as the owner, Mike Reeder, we are not related.
I came back and edited this the normal turn around I've gotten is three to four weeks. People like us are a side job to them they do gigantic stuff for NASA and offshore drilling suppliers, etc.