You can find some pretty decent stuff at garage sales, but (in my experience, at least) it tends to be more "modern but outdated" PCs and game consoles the kids no longer use than old computers dragged out from the attic. What I do, and has yielded a good variety of stuff, is find a local rag that offers free classified ads and put out a little want ad offering to clear old home computers out of someone's attic or garage. If you can make it sound like a service, it's quite possible you'll get something for free
