I think you're chasing the Westcoast tank. Be aware that they made them for both the 650 SX and the X2 and they are very similar but NOT interchangeable, so be careful you are getting the right one.
If you're chasing extra range it's possible to tap the oil tank into the fuel system (obviously you lose the auto-mix lubrication and run pre-mix). I'm not sure on the nuts-and-bolts of this mod so somebody else might be able to chime in.
This is going to kill you but I have one (Westcoast X2 tank) sitting unused (but I'm in Australia and the freight would be a killer). Happy to send images if required if you need to ID any tank you're offered. You may have to wait a little until I'm next at my storage unit.