A couple of ideas: I'm not sure of the water flow but there is a class II-III stretch in Jackson Hole on the Snake River that is pretty sweet. You'll have to talk to the companies around there to see if the level is good. If they say its too high for a newbie I remember a nice wave about half way down the run where some guys were getting pimpy. It was park and play. I know thats vague, but I'm sure some folks in Jackson will know what I'm talking about.
Also, in or around Bend, Oregon, there are a couple of good rivers. I would give a call to some outfitters in Bend and tell them your plight. I was visiting some friends a couple of years ago and saw some awesome stuff. too bad I wasn't paying better attention.
Anyway, that should help. You may be able to find some stuff in Idaho too.