I've had hundreds of vintage girdles and suspender belts where the rubber is missing or damaged. I am yet to ever see a rubber end to replace them with, so I've just had to replace the whole piece x how ever many clips you have hanging down. I wish I could tell you something more helpful.
As a result, I hoard them when I find them. My best advice is to look for damaged black garters and hope you get lucky. Finding the old metal ones is hard, and yours is unusual.