I think the original post is refering to the spreader arm support fitting, not the diamond wire attachments to the mast (ie "tangs" or "hounds"). Since the arm fitting must fit around the mast cross section, these are not readily available for non-beach cat places. Hobie will have it...but Murray's should be able to source it if you get them a part number. Cost will likely be a wash with hobie though.
BTW, you might want to ask what size rivets it needs...I've seen some 1/4" monels used there and those take a special puller to set.


Mike.