Farm and Fleet is a great place to start looking for such a item. Also Tractor Supply, sort of a larger general store for those living the rural life.
I would be looking for something under the bed, to keep the center of gravity as low as possible. Whine in the past, they could make gas and diesel aftermarket replacement tanks, the EPA is very stringent about fuel vapor leaks, so most replacement tanks today are diesel only.
TransferFlow.com is a great manufacture of replacement fuel tanks. I have heard that they make a replacement for the 26 gallon GMC trucks with something like 50 or 56 gallons, it extends a little ways under the cab, and is mounted between the cab and driveline.
Transferflow also sells tanks for Weekend Warrior toyhauler trailers. They also sell a toolbox combo unit, with up to 70 gallons in the toolbox.
Fred.