I just started making them, and to just see how they were I ordered the flattened bottlecaps with jump ring already attached from Rubber Nation, and got a sample pack of their necklaces, too.
I love the way they turned out and they were too easy!! Just added my image, topped with an epoxy dot, and that was it.
Of course, if I get into doing them in quantity, I'll probably have to start punching holes and adding my own jump ring to get the cost down.
ps- sorry for the bad photos