I've seen a few of these on EBay, both in cars for sale and by themselves. It seems there is some variety in what you can get. Some look professional, some are obviously home made. The one that scared me the most had a fill cap inside the trunk - which appeared to be leaking judging from the stain down the side of the tank!
It's an interesting idea, especially if you live a long way from a diesel fuel supply. But thank goodness for the popularity of diesel pickup trucks, there are lots of outlets around where I live and work.