There are any number of commercial plating shops throughout California - and you'll usually find them listed under "Industrial Plating".
Look also for Trophy Shops, Commercial Jewelers, and Religious Goods Manufacturers - as all have plating needs.
Black Nickel may prove interesting for them.
Failing that - you can chemically blacken the cleaned and polished brass with Liver of Sulphate.
Good Luck.
Scouts Out!