In the SF area I would look in Lattitude 38--several rail makers/SS fabricators advertise there. You might do a Google search also for the SF area also. Most likely this will be a custom job (Perhaps the people who do it for the factory will do this for you--but there will then be shipping and it may not be exactly what you want).
I have found with SS work it has to be finallized on the boat. SS will bend to some degree as the welds are heated, and even if jigged up, it may not fit exactly. We usually had the piece tack welde on the boat, and then final welded in the shop--back to the boat for final fitting--and there were some fairly large custom arches (10 feet wide, 10 feet high and with 3 1 3/4" pipes, plus davits etc).