Hi, Perry, and welcome to Cruise Chat.
River cruise operators that come to mind are Uniworld, Viking River Cruises, Peter Deilmann, to name a few. There are others. My suggestion would be to work with a travel agent who is experienced in river cruises, who can help you choose the right boat and itinerary to match your needs. But take a look at those to begin with, for a general idea of what's around.