if you cant find something closer to home, let me know and i will mail you the gizmo i used (2 pieces) .... one holds the 'cap' the other peens over the stem.
Jooper, any auto trim or boat top shop has the proper tool. It's actually a "C" shaped hand tool with different dies and such. Just install the proper die and snug'em up. There is a cheaper tool that is just a pair of vice grips and the dies slide on the jaws.
See if you have a store nearby called Tandy Leather Factory. They have ALL kinds of snaps and fasteners for leather work. Great place to visit, we have one up the street, its great!