Ask HN: Tips how to find cheap/free tech?

Does anyone have any tips and tricks for finding old laptops and similar items at very low prices or for free?

Are there any online platforms other than eBay or Craigslist where I should search? Additionally, are brick and mortar computer surplus shops typically worth the price?

I'm also interested in any tips and tricks for dumpster diving. It is legal in my area, and I would like information on the types of places where I might find discarded old technology.