how would you know which places to patrol, and when?

This is an extremely regional problem to solve. Where I am, which is a village and exurban-ruural, you would go to the electronics recycling depot and see if they have any choice items. Also you could call the various independent pc repair people to see if they have anything no longer supported but functional for free or cheap.

Also there’s various thrift stores that sometimes have computers cheap.

The closest big city is Vancouver so to curb cruise there I would pick upper middle class neighbourhoods with alleyways, and drive around on garbage collection days. I wouldn’t really dumpster dive unless I knew of a likely source from hearsay.

Most of my work is with Macs, and even one server is running macOS, so for those who don’t know how it works ‘over there’, one runs Time Machine which is a versioning system keeping hourlies for a day, dailies for a week, then just weeklies after that. It accommodates using multiple disks, so I have a networked drive that services all the mac computers, and each computer also has a USB drive it connects to. Each drive usually services a couple of computers.

Backups happen automatically without interruption or drama.

I just rotate the USB drives out of the building into a storage unit once a month or so and bring the offsite drives back in to circulation. The timemachine system nags you for missing backup drives if it’s been too long, which is great.

It’s not perfect but very reliable and I wish everyone had access to a similar system, it’s very easy, apple got this one thing right.

I have a bunch of old macs here with different distros onthem, mostly Mint, that I have been trying to give away to locals (without being obligated to provide support, which is the stickler apparently). They all run great. One could dumpster dive or curb cruise, or around here, lurk at Recycling.