The steel L bed tanks are around $450 at the local ag supply places here. Get one of those, buy a pump and nozzle, and you have a cheap setup that can also fill other equipment. That's what I have in the back of my truck.
You do have to be careful about used steel tanks, as they can rust. If they are empty, pop the fill cap off and look around with a flashlight, or better yet a borescope. But if they have been taken care of properly (mostly stored full), you can find them without rust.
I've never had my main tank expand and leak after topping off, but like most, I fill it up then drive home. The bed tank, on the other hand, will absolutely leak a ton if you don't leave some head space.