Any tips on finding cheap used ones, other than scrounging eBay and the like?
Find a brother in law that works in IT. I changed his brakes and he gave me three Optiplex micros that were e-wasted from his work.
eBay, Amazon, sometimes you can snag deals when they release newer versions, they actually had a few on Black Friday/Cyber Monday.
Tbh, I got mine in a pack of 4 on eBay (I only needed 3), cleaned them out a bit, bumped the RAM and put new disks in them and they’ve been solid.