Ogmios

Could it perhaps be specific models that are no longer manufactured? I was checking out the price I could expect for one of my old PSUs and found that it was apparently a particularly well liked unit for some reason, and so it’s used price was a fair bit higher than expected.

They actually are open source, and there is indeed a lot of material out there to help people with everything they need to build their own. The only part that is illegal to make on your own is the part with a serial number on it, for tracking purposes.