...you could also look at the tanks at Coyote Gear
and while their tanks are similar to the one Mr. Minecraft
linked to above, they also sell tank PARTS like the filler necks so if you have any welding skills you might could make your own tank.
...here's a link to the page where you start by selecting the size first, ...take a look around, you may not need much of what they have now but down the road, who knows.
...oh, and here are a couple of pics of my bike and the tank i bought from Coyote Cear.
...as you can see, my tank is also a keg style tank.