Rarer than 5 speeds.
Here's a link you probably found but here goes:
http://www.sportomatic.org/
I think the rarest sporto would have to be a 914/6. I think there's one in Holland. There may be just a handfull.
I test drove a sporto (targa 70's model long hood) in college with a comfort group suspension. It was like driving a Buick Skylark. It was relaxing, very cushy. The wheels were rather tall-60's series.
I'll bet sportos are hard to find parts for and $$$+ if you have to buy parts.