Here is my experiance.

Good luck finding one that wont cost an arm and a leg. I ended up making a fiberglass tank for my cantiliever Schwinn. Unfortunately I missed the "caution" on the Kreem sealer instructions and turned my tank into modern art. On to round 2 and useing Caswell for a sealer. There are plenty of different tank ideas here from juice tanks to old moped tanks.

It's highly unlikely that there's a prefab one that happens to fit in a cantilever beach cruiser, even if it's a commonplace model as it's a somewhat unusual shape for a tank... I could be wrong, but I don't remember seeing one. Tanks are usually generic, a "one size fits all" like the kit tank, a behind the seat can, or an under the rack box. There's also the custom made tanks some offer, but those are usually premade for a very specific & limited selection of frames. You can ofc contract one made for your bike from a local metalworker... but that gets pricey quick as it's a one off, with the extra time & mistakes that entails.

but...

I really think making one is only as difficult as you decide you want it to be, yet still worth every effort