There is no magic to it, just plain old hard work - like everything else. Of course you keep an eye on the usual used stuff sites (Craigs list, Kijiji, ebay, whatever else local, auctions), and talk to people about your interests (car, air plane, train clubs, Maker fairs, . . . ). Develop and care for your own net work. I assume you have some machining kit and ability? Do little freebee jobs for people - sort of a "for future consideration".
I am always working on my machining stuff. I never really know what is going to show up next - but weird bits and pieces of different things are showing up constantly. I often go and look at machines that I have zero interest in purchasing, because often there is other stuff that wasn't advertised. I talk to the sell and often that gives me other leads. Once again, I don't know what may show up, but so what - cool and interesting stuff is showing up all the time (always cheap, often free, usually needing some repair). Things that other people are throwing out is probably my #1 source (grinders, drills, vises, drill bits, chucks, saws, tool chests, machine lamps).
Tracking down good kit for cheap is just work, nothing special, no secret tricks, no special web sites.
