My vote is to build your own tank out of aluminum. It cheap easy and easy to fabricate. I have also made the suggestion to get a keg of beer and keep the keg. Its already sealed and you can have fun drinking it rather than building the tank from scratch. You will loose your deposit, but its cheaper than buying a tank.
One problem with junkyard tanks is they tend to get wet and often are close to having pin holes without knowing it. The shapes also suck to work with and look stupid mounted out of place. You may find yourself mounting it with rope, because you can't weld to it and its not made for your truck.
I built one out of 1/8inch aluminum if you want to see a couple of pics:
Pics of custom tank You will see that you can make some crazy shapes. But for the most part, you just need a rectangle.
