I don't know about price for molds. And you have probably answered you own question by now. I haven't seen molds very many places.

You can buy a concrete tube at any box store and then cut it to make forms for round stepping stones. I use iron oxide to make my concrete darker, and I embed them with creek pebbles. Look for a good sand mix. You can get real good results if you add acrylic hardner to the mix.