L.A. has a lot of good flea markets - the Rose Bowl is always a good one. I have taken to eBay, too. Some of my favorite things - especially chairs, for some reason - I got on eBay.
I have a weak spot for late '60s-early '70s yippie paperbacks and protest manifestos. I find them at flea markets or online. One of my favorites is 'Right On,' a compendium of student protests made into this 95-cent paperback with the most amazing graphics.