Try Home Depot or Lowes. Also call places that supply "Bars". I presume you are looking for a deeper sink that isn''t very large in top dimensions. Have you called a boat manufacturer (emailed them) and asked them where they get their sinks? Just a couple of thoughts.
Defender has sinks in their catalog and Im sure that West or Boatus does or would special order them for you. Not sure if your typical sink designed for the home would fair very well in the marine environment.