Quoted from Elfman:
Patience is also key. If Dr. Who is what you want, then you will have to probably wait for it. Try not to get frustrated when one doesn't pop up right away but just be ready to move when one does (have wheels and cash ready).
+1. This is a mistake I made when I started. I bought a Cyclone, but wasn't crazy about it; really wanted one of several I had on a list, but that wasn't it. Eventually, a High Speed and a Funhouse popped up in my area and it was worth the wait! It can take a while, but it will happen.