I'm in a similar situation in life (young and poor), and I've fdound a couple things that work: ebay and good old luck,
Look at ebay a LOT. I look every day, just to check what cars have come on that day. if you're after a certain car (i.e. Superbee), then look for that, but also look for Coronets (or whatever the base-model of your car is). the big name cars will go for a lot more, and if you're on a budget and not too picky, a clone will do just fine.
Secondly, there's just hunting. newspaper classifieds are always good, but there's something to be said for driving (or walking or biking) down streets you've never been down, and looking in side yards. I'm from a small-ish town, so eventually I looked down every street in town, and found a couple jewels.
Do you know an older musclecar guy? talk to him, see if he knows of other cars. THAT's when you'll find the cars in the backyards too.
good luck, just keep looking and looking, if you're really interested, you'll find one.
~Erik~