Do you have the under-floor tank that fits into the frame on the outside under the dinette? I don't know what the other folks have who are mentioning where they bought their replacement, but as far as I know Scamps (and maybe some Bolers) have a squarish tank that simply sits inside one of the bench seats vs. the one under the floor, which is wide and shallow.
My 1974 has the tank under the floor.
The under-floor tanks like in my Boler (and maybe yours) are available from a place called Polyrama Plastics, who made tanks for OEM use by Boler. I believe they are in Edmonton, AB, so you may be in luck. I looked into buying a tank from them, and, although the tank was reasonable, shipping to the SE US was a bit prohibitive.
They were responsive by telephone and e-mail and knew about the Boler tank.