Whole seat belt and retractor are one assembly. I've got to do this myself as I have to manually feed the belt back in when I unbuckle and get out. Partznet has them. Item number 1 on the diagram. You'll just have to make sure you're getting the right color (either stone or dark grey, not sure which) and make sure the part number is for the LHS (driver) of the truck.
2006 Toyota 4Runner Parts - Conicelli Parts Center