I had such an experience today. My house was built in 1984, and a bathroom sink faucet recently developed a slow drip. Figured that it would be an easy fix by replacing the cartridge. So I removed the cartridge (which surprisingly was extremely easy to remove), and set off to the Home Depot to buy a replacement. No luck. Lowes was the same. I didn’t have time to go the the independent hardware stores, so I planned on doing that tomorrow. But I decided to do a bit of research this evening. Turns out that the faucets in my bathrooms are some off brand called Spa, and it looks like that company went away about 35 years ago. So it looks like I’ll be installing a new faucet this week.
I will say that while it would have been nice if the builder had used Delta, Moen, or Price Pfister faucets, I am impressed that the Spa cartridges were in service for 41 years with no issues until now. That and the fact that removing them after all this time was so easy. The bonnets holding them in place unscrewed as if they were installed yesterday. All for naught since replacement parts are not available.