It's like all of their other planes and trying to guess where the "specials" will show up. They are scattered all throughout the route system.
About the only thing I can tell you with any degree of certainty is that you WON'T see any -200's at SNA (John Wayne/Orange County).
Are you looking to fly on one or photograph it?
It's a bit of a crapshoot at best. But if you are looking to photograph them, your best bet will be to try at their biggest cities....
LAS, PHX, LAX, DAL, etc.