To look for bargains (at Circuit City, where I buy my games), check for a wire bin. Since they stuff everything for every system (I've even seen DVDs in there) you will have to look around. This is how I found MLB 2005.
I remember seeing a store called "FunCo Land" at a shopping center. I always thought it was one of those party supply places until I looked inside the window one day and saw a bunch of old Nintendoes sitting in there. I guess this is like a used car lot for Nintendoes and games. The store went out a few years ago before I ever was inside of it.
Are used games safe? I would worry about the reliability of one coming from someone who wanted to get rid of it just like that. I don't think a Nintendo can get a virus, but is it possible?