First - Gravity flow diesel aux tanks are completely legal.
I full time and have had an RDS aux tank in my 2013 F350 since new. I have the 40 gallon/tool box combo. The OEM tank is 37.5 gallons so I have over 70 gallons total, depending on how much fuel I put in the aux tank. The thing is great.
The gravity feed works fine and their install kit includes a 1-way solenoid that stops flow from the aux tank when fuel backs up the OEM filler line and contacts it.
RDS tanks are sold through the company itself or Northern Tool. RDS has many sizes with several different footprints - combo units like mine and also ones that are fuel only. Not sure that the Northern Tool site shows all available options. I bought mine from RDS.
I like mine because it fits below the bed rail and I can still use a tonneau cover when not towing. If I had to do it over again, I would have bought their 60 gallon combo as it has the same dimensions - just a smaller toolbox. I just don't carry that many tools in the box.
I suggest you take a look at the RDS site and browse the various options. You can also call them to see if they know of an installer near you if you don't want to do it yourself.