The best place to find free stuff is.... knowing people, especially extended family or in-laws. For example, my mother-in-law keeps an eye out for me all the time. Just last month(?) she was driving and saw a bunch of computers by the curb being thrown away. There was an old Compaq and Dell in the haul, one of which contained a PCI Voodoo 3 3000 and AWE64. Then she tells me that her brother wants to get rid of his Commodore collection. She was talking to him on the phone (he lives about 200 miles away) and he said he was going to throw it away but wants to give it to me instead because he knows I collect retro computers. I still have to go pick it up, but supposedly there's a few C64's, software in box, peripherals, and several other "newer" Commodore computers.
Just let people know about your hobby, and good things come. I've had co-workers donate things to me that their wives told them to throw away.. GeForce 2 GTS, nForce motherboard, beige AT case, beige ATX case, dual socket 940 Opteron motherboard, etc.
Free stuff doesn't come often, but some people simply just don't want to go through the hassle of selling, don't care about computers, find no value in this stuff, or just want it out of their house to make room for more important things to them. You can also give people a monetary compensation as gratitude if they will accept it.
I'm still waiting on a storage closet at my local high school to be cleaned out, because there's a mint condition complete Amiga 4000 Video Toaster setup that was stashed away. I used it back in high school in our TV studio to do non-linear video editing with. It performed video transitions, 3D graphics, title pages, and chroma-key overlays. There were very few people who used it because this was in 2000-2004, and nobody knew how to use the Amiga OS. 😵 I was the person who put it in the closet once I built the district all brand new non-linear editors running Adobe Premiere 6, Canopus EDIUS Pro, and setup a GlobalStreams Trinity to replace the Amiga.