Don't know what you mean by
"cap on the OEM container is also the valve and a 3/8 hose fitting combo."
Valves and fittings are very inexpensive, so unless I'm missing something that shouldn't be a reason insist on a direct replacement. in many cases you can just swap the fittings to the new tank.
Bladder tanks are ok for water, but they only have an 15 year lifepsan...yours seem to have failed right on schedule. A top quality rigid plastic tank will last at least as long as the boat...and cost a LOT less than comparably priced bladders...and Ronco Plastics is your best source
Ronco Plastics Marine Catalog
They make TOP quality water and waste tanks for a very reasonable price and have more than 400 shapes/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.
However, if you're still determined to go with new bladders, Nauta and Vetus are the two mfrs who make most, if not all of 'em. Google 'em to find out what they have where to buy 'em.