I vote for making your own tank on one of the problems.
is a link that goes into "The Lost Foam Technique" a bit. There are some good videos on youtube too if you search them out. Once you get all your layers on and wetted out, vacuum bag it to get all the air and excess epoxy out, search for youtube vids on that as well. Should be strong and light, and mostly, any shape you like.
I would make it several layers of carbon fiber thick, and cross up the laminant layers a bit, 0, 90, 45 type of schedule.
My only issue with it would be to use an epoxy that won't break down with the gas in it, somebody tell me that's possible. I think it is, because I'm pretty sure I've read about fiberglass boats having molded-in fuel tanks...
In the end, you can probably gelcoat it any color you wish too.
If you're at all interested in this, I can give you links to some products I've hunted down (when I get home). PM me in case I forget to check back here.