Depends on where you are located. That's a pretty common item, should be available at any marine store, or catamaran dealer. Fill out your profile (click the profile link in the main menu) and folks can help you better.
In a bind on any of the Prindle Cats and the Nacras with booms, They all use the box between the boom and attachment point on the mast. A 1 1/2" X 1/4/20 HEX head bolt works fine with a nylock nut. I would normally take one of the pins out and replace it with a bolt to not lose the box.
Kirk Newkirk