The hatpin category on Ebay includes many old or vintage items as well as new.  Hats have been worn in the military and by many other organizations.  Any of those groups could and probably did produce a hatpin as a fund raiser or gift for their members.  In my own experience, the Albuquerque International Balloon Fiesta, has sold a variety of hatpins over the years. 

The list below shows some of the hat pins for sale now on Ebay.  You will need to narrow the search to get to a manageable number of auctions.