The tanks show up on ebay from time to time, and they are 2.5 by 7 inches give or take. If you got that WB with the cast aluminum tank mount, you scored big time. They are not easy to find and when you do, you pay for them. If you don't have that piece, you have to make one- basically a cradle. I dont see one of those tanks on ebay right now, so the auction must have just ended.
Don't pay much more than $40 for a NOS tank.