There exists a larger OEM tank, but the are very rare! Your best luck is to search ebay.. Think they were standard on Australian bikes.. Here it is, but it's kinda expensive.. http://www.dirtbike.at/husky/te/te_tanks.html
I used to have one, bought it for $250 USD and sold it for $250 on ebay. I agree, just keep an eye out on ebay.
Found one on Craigslist years ago. Bought it because I knew how rare it was. It was not a "normal" rare big tank, but one of a few larger clear tanks Acerbis made just for the Incas rally in the early 90's.
I have one that I have not used in years....so I would be willing to sell it.....but I would want bank for it