What is your best resource for second-hand fantasy literature?

I went to a second-hand shop today to look at the books they have collected over the past couple of months. Often it is hit-or-miss in these shops. Most books there seem to come from people cleaning their parents' house when they have died. Because the fantasy genre has not been mainstream, these "inheritances" almost never have any fantasy related books. But if they do, there is often a whole bunch of them and quite an eclectic selection. And that is why I keep going to these places: You never know what you might find next!

What is your best resource for interesting second-hand fantasy literature?