I can tell you that in Wisconsin you rarely see them driving around. Once in a while you will spot one. The exception is when there is a car show near by. Then they tend to come out in bunches as long as the weather is nice.
I would imagine that they are more common in the southern states since weather is a bit nicer and they have a longer driving season.
As a point of interest, I was in Kansas City last week and came across a classic car lot. They had a lot of late 40's and 50's Buicks, a couple pick-up trucks, and 2 Saabs. But what really caught my eye, (and made me think about how to get them home) were a 1948 Chrysler Limo and 1957 Lincoln. They were both in need of a lot of TLC but were good solid cars to start a restoration on. The Lincoln was around $2000 and there was no price on the Limo. If I didn't already have projects waiting though, they would have came home with me.