Nobody, but NOBODY in the general aviation manufacturing business does a "special" that the manufacturer must make specific for their product unless there is no way to use a generic OR automotive product with a "PMA" stamp put on it by the manufacturer. Sure, metalwork specific to the brand, hoses cut to length, and the like are manufactured. Most electrical parts are off the shelf from one manufacturer or another ... relays by Potter, starters by Ford or Chrysler, landing lights from Chalmers tractors, and all the rest of it.
If your A&P/IA is so paranoid that they won't let you do the legwork to find the exact replacement in a catalog and use it, then they need a refresher course on 21.303 (b)(2). All you need to do is draw the part up and then tell your NAPA or Digikey parts house you want part number A12345 from Boozefoo Manufacturing. Perfectly legal. Owner produced parts.
Brien, you are DEAD wrong. Sorry. And yes, I've had my A&P (& IA) for 50 years and so far the Feds haven't asked for them back.
By the way, relays normally don't "go bad" without being repairable, which is also legal. If you can take it out of the airplane and put 12 volts on the coil and hear a "click" then the contacts have gone TU and that is a ten cent spray of contact cleaner to fix.
Now I would NEVER recommend this as a fix, but if you spray the contacts with cleaner and then hook the relay up on the test bench with the coil connected to the normally closed contact, it will become a "buzzer" and clean the contacts in a couple of seconds. But I only know this because my friend Ernie does this, and I would NEVER do this to a certificated part. NEVER, do you hear me?
Jim