I tried to find them in mongoose as well, so I used other rules like T4.
Don't have it in front of me right now, but dismount-able tanks come in two forms. (per T4)
The first is simply a large rigid tank that bolts to the side of a ship (or in a cargo bay) and you can pump fuel directly into the drives. When you remove the tank, you unbolt it, and move the whole thing as one large tank. I think they are 5000 credits per ton.
Then there is the kind I use in all of my designs. They are like a walk-in freezer/cooler in a commercial kitchen. They can be dismantled and stored at the back of a cargo bay, taking 25% of their full space, disassembled and stacked out of the way. They too can pump fuel directly into the drives. They cost 7000 credits/ton and take 1 man hour/ton to remove/install.
Not sure about cost of collapsible, "fuel bags" but they take up 15% of their original space when stowed away, but cannot be used to pump fuel directly into the drives, only refill the normal tanks once they are drained.