Try digikey, go to
www.digikey.com and search for "tac switch", now you need to find something I think about 6-7mm tall in order to poke through the shell which is somewhere betwee 2-4mm tall depending on where you drill plus maybe an extra 1mm if the tac switch doesn't lay flush with the surface. There will be a million options but all are cheap, order more different models than you need and try them all out. Since some of what you order will end up not fitting right :-(. Anyway the parameters to check:
Diameter of the button
Height of the overall button
Stiffness of the button (they put this in grams or newtons, I believe our tac switches are 160g stiffness)