Check three places. First the Portabote website. They have gotten some credable press from Practical Sailor and others.
Next look at two clubs on Yahoo. One is California portaboters and the other portaboters northeast.
I have read a lot and am convinced they are a good idea. For me 10'' and a 4hp motor.
You might also look at Traderonline and Ebay for used ones. Genesis 3 and 4 have square transoms for motors.