Where I could get a free water holding tank or if there is a name or model number on your tank, mine was removed by a previous owner.
I don't know any sources for free water or waste tanks, but Ronco Plastics
Ronco Plastics Marine Catalog makes TOP quality thick-walled water and waste tanks for a very reasonable price (they sell direct) and has more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank. Be sure to specify "water" or "waste" when you order the tank.
Fwiw, Ronco has made OEM tanks for
many boat builders over the last several decades. I don't know if Oday was one of 'em, but if they were, they're likely to still have the molds for 'em. Definitely worth asking.
And btw...although the standard toilet discharge fitting is 1.5", Jabsco is the only mfr who used a 1" discharge. They finally began offering 1.5" too...so if your toilet is a Jabsco, the discharge fitting can be either 1" or 1.5". Because holding tank fittings are all 1.5", a 1" would require an adapter in the discharge. If your toilet is a Jabsco with a 1" discharge, the best solution would be to call Jabsco for a new 1.5" discharge fitting. That also applies no matter what size it is on any toilet if you have a straight discharge fitting where a 90 is needed, or vice versa.