I'd try D&R Marine, Rudy will know what tank was there originally. You could use a colapsable 5-gallon water carrier from a camping store, yo umay need to replace the included cap/spigot with something else to avoid the cap leaking around the valve assembly in the original cap of those tanks. (we were lucky enough on our CAL 21 to have an extra cap without a spout that we simply added a thru-hull to that fit our water line.) I have added pics of our tank setup, I wish I had a pic of the actual tank cap. The "Y" fitting allowed us to fill the tank without disconnection the line to the pump, we filled the tank with a hose from the dockside faucet (admittedly not a perfect solution...but common on many small boats) which we attached a quick-connect fitting to. That plugged into the tank inlet, we opened to valve on that branch of the Y and then opened the valve on the hose, when the tank was near full, we would shut off the hose valve, disconnect the hose, squeeze the tank to vent out the air that came in with the water, then shut the valve on the fill side of the Y. Believe me....it was a lot easier to do than to describe! Oh, and yes....I admit....I did dampen the cabin once or twice while filling the tank...oops!
PS: that blue box covered an electrical (12V) buss bar.