Making your own stepping stone molds is a simple process: just build a wooden pour box, put your model stone in the bottom of it, and cover it with pourable mold-making compound, such as urethane. The question is -- what to use for a model? Here you are limited only by your creativity. Your model stepping stone can be almost anything you can fit in the box.
The most traditional stepping stone look is that of flagstone, and they're the easiest mold to make. Just put a flagstone in your pour box and cover it with mold compound. If the bottom of your stone isn't completely flat, you'll have to set it in a bed of modeling clay so that the urethane can't run under the stone.
You can make stones that look like a cluster of bricks by arranging paving stones in the bottom of the pour box. If you want to simulate a grout line between them, you can mix up actual grout, lay it over the pavers and trowel the excess off. Let the grout harden before you pour the mold-making compound.
Place a 2- to 3-inch-thick slice of log in the pour box. It's a good idea to use a just-cut slice; this way the bark will still adhere tightly to the log. Seal the log with acrylic sealer and let it dry before pouring the mold-making compound.
For a whimsical touch you can make stepping stones that look like the prints of an oversized bare foot. Trace a large footprint on a 2-inch-thick piece of expanded polystyrene (e.g. Styrofoam) and cut it out with a hobby knife. Seal the foam footprint with clear acrylic sealer, then cover it with a layer of wax mold release. Don't skip this step -- unless it's properly sealed, the casting resin may melt the foam.