At www.motel6.com you can search for motels along a route (Reservations > Trip Planner tab), and you can use coupons to get a 10% off (check http://www.retailmenot.com/view/motel6.com)

Also, google maps is a very useful resource when looking for motels. Search the location you want, then search for the word "motel" and it will show you all motels/inns/hotels in the area. You can now zoom in/zoom out/drag around the map following your itinerary and it will keep refreshing the motels for the portion of the map you're viewing.

Near Bryce, I found the Galaxy of Hatch, $53 including taxes for a double room.

Between Zion and Bryce, Mount Carmel Motel and RV Park, 1 queen bed $40 plus taxes.

Haven't been to any of these motels, just sharing what I've found while planning my trip, so I don't know about their quality...