u will probly have to go through the race department or your mabe your local dealer. if al else fails, have one made out of aluminum. Pro motocross teams do this a lot during the longer tracks at the nationals.
the factory doesnt make a big tank kit for the 04' trust me i got one, but there is a guy that is making a bracket to mount the tank down. his # is 218-393-5535 ask for kelly he selling them for $50 dollars they come powder coated, you just need to buy a big tank.