It is available from CSS, but the trip we went on last year sent us back up pretty much to Montego Bay (I've seen that it's offered closer now, but don't know if they've switched locations). We weren't overly impressed with the trip, because there was a pretty hard sell from our tour guide to buy carvings he had done. You can't just say no thanks and walk away while on a raft. Also, this is a very mellow river - not an adventurous raft trip if that is what you are seeking. I would say there are better excursions available.
We enjoyed it very much. Our guy wasn't all that pushy. It was a nice relaxing ride down the lazy river. Just go to the Tour Desk when you get to the resort. It cost $90 each, which included the trasportation and the excursion. The raft captians usually get (and expect) a $20 tip for the ride also.