The most important advice I can give you-
The moment you find yourself saying "No more games, I'm finished!" is when you know you're addicted

Next thing you know, you'll find yourself with no other furniture, unable to move, and catonic in front of a joystick shouting to the outside world that "I CAN QUIT ANY TIME I WANT!"
But even as addicting, expensive, and time consuming as this hobby can be, I would say it's definitely worth it. You've made a good choice. If you play your cards right, it can be cheaper than modern gaming. I spend less a year on the machines (usually two or three a year as well as all repairwork and extra parts) than most of my XBOX/PS3 crowd friends do on their gaming.
I would suggest starting out on eBay. It sucks for sellers, but as a buyer I've never been disappointed. Craiglist... don't be surprised to get some very "interesting" experiences. With eBay and their overly-buyer-centric feedback system, you're usually going to get a good experience because the seller doesnt want you to leave them a mark on their record. I've only worked with people at at least 99 percent feedback and haven't had an issue so far. However, the best deals will come from connections. It's amazing what percentage of my incredible deals (and one free, working machine!) came from connections in the hobby. We're a good bunch of guys, get to know the KLOVers in your area a bit better, go to their shindigs or host one of your own, and you'll find yourself with some steals eventually.