Learn your locale.
See if there's a computer user's group (it's pretty bare around here, even though a major metro). Consider that Ham Radio and electronics buffs often have lots of computers (and tend to be older). Talk to people. Ask.
Talk to people at thrift stores even if the stores are bare. I've had people say "we don't deal in that but we have a few things in the back that were donated."
Talk to recyclers. They almost always say no, but I've made connections through them. I was talking to one up in the hills a couple weeks ago and he said "hey this guy I just talked to has a couple of storage sheds full of stuff" and I ended up with 10 vintage laptops and a few motherboards a week later.
The problem is that stuff is a little too expensive for casuals (like me) and way too expensive to ship, so it either sits in a shed or attic until it rusts or gets shredded. But there's a *LOT* of stuff still in shed and attics, you just have to be really active to find any of it.
I was thinking of trying to start a club just hoping to find some stuff, but I'm terrible at crowds and meeting new people and so on. Kind of like (surprise) a computer geek. If I had a little more money there's a large urban area a few hours away that is flooded with old hardware, it's just a little beyond my budget to spend a few days digging around there.
If shipping was a *lot* cheaper, it would be easier for all of us. And if we were all rich, handsome (or beautiful, your choice), healthy, etc.