If you don't mind reproductions, I've found the folks at Worldwide Militaria Exchange to have the best prices and selection of regimental cap badges. http://www.wwmeinc.com/mm5/merchant....ode=10-002-030 there are six pages so don't overlook the entire list.

The Scottish Reserve thistle badge on p 6 is a really handy badge to own. It doesn't look so much like a military badge, so it's useful for anyone who wants to sport a Scottish cap badge without any particular clan affiliation. I got a couple to wear for general purposes. They also have some nice Irish badges, at least one of which works for a non-military look as well.