I know of no Tundra specific replacement larger tanks. Most of those are made for Ford, Dodge, and GM as they are obviously more prevelant in industry use. However, there's tons of in-bed auxillary tanks available. Here's some links.
AeroTanks-we specialize in auxiliary fuel tanks.
ATTB - Auxiliary Fuel Tanks
Aftermarket and OEM Fuel Tank Systems
Northwest Metal Products - Since 1977
With the aux. tank in the bed of my wk truck, I have an 800 mile range and it's great. I'm thinking of building an auxillary tank to go underneath the bed of my Tundra, as I have a longbed and there's tons of room down there. You could also have a local shop build you one to your specs. There's nothing real technical to them. A 26 gallon tank is useless, and makes no sense when there's clearly room for a 40+ gallon tank under there.