eBay or Facebook marketplace usually

I'm a little out of the loop, but a few years ago a lot of ex-corp stuff tended to get dumped on Gray's. Might be worth a look but I have no idea how enshittified it is these days.

I usually browse craigslist, but in larger cities there are usually companies that handle warehousing and selling of old hardware when company IT does a full office upgrade. FWIW, I use old workstation hardware because it is usually less finicky and power hungry than rack hardware.