Best bet for me has always been to get along to Hardly Normals or Good Guys and ask where they keep their trade-ins. More often than not they are all out the back at the delivery entrance. Have a sniff around see if anything takes your fancy and then ask the warehouse manager if he minds you taking one away. They usually turn a blind eye. Just dont expect it to be clean or to get any guarantee.
It also helps to take a few bottles of homebrew along, as it works as well as cash!