With patience and from eBay and local second hand websites. If you’re in Europe you’ll usually see sellers from Germany selling them for cheap, in the US there are a LOT more offers.
Regardless, like used cars, sometimes a specific generation that is cheap today can be more expensive tomorrow , it all depends on the amount of machines someone or some big company is dumping at the time you’re searching for. In my case I can usually get things locally cheaper than eBay, for eg. recently I saw a very good deal on a HP Elite Mini 600 G9 i3-12100T 16GB of RAM, NMVe 256GB for 300€.