Transfer-Flow is one place that sells DOT rated tanks that are legal to use on the road.
I know DOT standards got a lot tighter these past few years... in the past, just a simple fuel pump between the two tanks would do the job, but now, there are other issues, and oftentimes, the only pump available for a pickup truck model is one that sits in the bed, and has its own fuel nozzle. To use it, one has to stop, pull the nozzle out, put it in the fuel filler receptacle and hit the pump lever like one does at a gas station.