One must be CAREFUL when chosing a used drysuit. I bought/sold 3 used suits before breaking down and buying one of the next-gen suits bubbles mentioned. It was great for about a year, until I found a sweet DUI flx 50/50 (the previous owner used it approx 20 times, grew sick, and passed away (not in the suit)).
Your body-shape will play a large part in the availability of suits from which you can choose. If you have an "average build", you'll find more suits than someone who's shaped a little more... uniquely.
What you must first know, however, is what type of suit you want. Do you want crushed neoprene, trilaminate, a mixture of both? They all have their good/bad points, as well as different diving characteristics. Just like when buying anything else that's used, WHY are they selling it? How many dives does it have? What kind of condition is it in?
Your LDS may be a good place to start. They may sell their rentals every so often. One of my good dive buddies lucked-out and snagged a DUI suit for pretty cheap. It's required a little bit of work, but overall it's been cheaper than buying new.
The thing that will help out the most though, is patience. Be patient, watch the SB classified. In this economy, people are selling all kinds of things! BTW, depending on your build, I still have my next-gen pro dry. I'm looking to sell it, the bag, 2 undergarments and a pocket for cheap.
Hope you find one!