What exactly do you need, the whole fitting? Assuming it's similar to a 278, I think it's mostly just a quick fit hose fitting on the outside attached to the Whale brand connector/pipe on the inside via a standard plumbing part. A trip to a plumbing shop/hardware shop with the existing parts should enable whatever you have there to be rebuilt although there might be something in there that restricts pressure.
The part number from my manual is "Fitting, water inlet 035-0027".
The units appear to be called Water Pressure Regulators on both defender.com and great lakes skipper online stores, "Inlet water regulators are designed to protect your boat or RV water system's plumbing and fixtures from the possible high pressure damage that can be done by an unregulated municipal water systems".
I found them on Amazon also, "Jabsco 44410-1000 Marine Water Pressure Flush Mount Regulator (35-PSI, White)".
