My approach which got me some success is to search my local classified ads for branded PS/2 to USB adapters/converters.
There are many common generic ones and I actually bought one to try out but the beige generic one I bought only worked on only one of my three PS/2 keyboards.
The 3 branded converters I bought (a Belkin F5U119-E, an Aten UC-10KM and a Radioshack 26-724) all worked on my three PS/2 keyboards.
I did not want to pay overseas shipping for small items like this so I spent some time scouring the local ads including FB Marketplace and I got lucky to find nice branded ones at reasonable prices. The Aten and Radioshack came with their original packaging and are in like new condition while the Belkin appears to be in lightly used condition.
These are the 4 converters I got currently, will likely buy more if I find similar branded ones in the local ads again:
https://imgur.com/2vTifknThe Aten UC-10KM is a tad more convenient to use due to its longer cable:
https://imgur.com/jwtHcts