Probably the best way would be to befriend one of the workers at these places, explain what you are looking for and "help them out" with a "finders fee". That way you know they'll stash the good stuff for you behind the counter. Just explain well what you are looking for so they don't call with the wrong stuff.
I think though that recycling centers, schools, large company IT departments, etc. are probably the best places to check these days.
I got really "lucky" with a post in an online linux users group forum, from which one response netted me over 30 boards for about 30USD. But after all the work of cleaning the things up, which fortunately I found fun and relaxing, I realized that from a financial standpoint I would have been better off ordering 3 or four boards I was interested in from ebay even at premium prices. The benefit of finding all these old boards though was that I got to try 5 different Alps switches, Cherry blacks, a Mitsumi switch, and a few other odd ones, probably about 10 switch types in all. Alternatively for this though, it would be great to have Geek hack meet-ups.
My case would best be described as "overdose", which helped me to get switch-lust out of my head a bit. In the end I found the one I like the best (Alps Yellow linears with cut springs) and plan to stick with this for a while, until maybe I try a Topre based board.