Buying a fifth wheel with a tow vehicle.

I don't think there is a specific site that sells combos. Check all the usual places to buy FW's (CL, RV Trader, FB etc. ) Personally I don't think shopping for a combo is the right approach. You are severely limited to a small percentage of the market compared to the FW's and trucks for sale separately. And in the end you have to love both at the same time. My advice, Find a FW they like with the floorplan etc they want, and then find a suitable truck to pull it. That way you get what you want instead of just settling for very small market of combos that's available.