We have never had difficulty getting a houseboat at Powell.
I assume you are going to be launching out of Wahweap. So your 2 options are these.
http://www.azmarinas.com/AntelopePoint/ http://www.lakepowell.com/houseboats.php Yes, there is a monopoly......
We have stayed on the Discovery and Voyager boats and thought they were fine.
Costco has a contract with Lake Powell and houseboat....you may check their prices as well.