You try the buggy places - suppose you gave ebay a shot already..Aluminum right? Mounting in the bed?
Well, not sure on the price -anyway - truck hound has them as small as 3 gallon -
http://truck.hownd.com/product-1963-...fuel-cell.html
Ebay has them for $69 - 5 gallon -
http://search.ebay.com/truck-fuel-ce...ageNameZRC0024
If this is what your after. Hope it helps.