There are dozens of salvage yards and used boat parts chandlers (like Sailorman in Lauderdale and "http://www.boatersresaleshopoftexas.com/" ) throughout the US. I suggest you get on Google and search them out and contact every one until you find what you need. EBay works, too.
Sometimes it takes weeks or months of constant work, but I rarely don't find it in the end. My last score was a Jabsco/Par diaphragm pump discontinued years ago, in almost new condition, for a C-note.