I get some old PCs that have been retired from a college for really cheap prices. They only have a couple and sporadically throughout the year (More towards the end of the semester) but you can get stuff thats really cheap. Another good places to find hardware is in ditches (Surprisingly). I have found computer towers with motherboards still intact in ditches along with old TVs (That's what I mainly find)
The laptops go for 25 each without a hard drive. Oftentimes they come with only one RAM stick when they are supposed to have two (The idiot who was putting these up for sale took the stick underneath it but not the one under the keyboard) They have all sorts of cheap stuff there. There are also some other places but they don't upgrade often.
I have found some networking gear at my local thrift store. Other than that eBay and Craigslist are usually the first place I look when looking for used stuff.
My university is having a major clean up right now and behind the building is a pile of old computer tech (both collectibles and just useful stuff). Me and a few others managed to get some awesome power supplies, and mechanical keyboards. The rest was too old and obsolete.