I would think you could just use an ice cube tray or cupcake wrappers to make tarts. I have never seen a tart specific mold but I haven't looked either. I think the tarts are pretty expensive. My mother in law loves them so we always buy them for her.
We live close to a hospital whose gift shop makes the candle tarts. I'm usually able to pick them up when I need them for guests or as gifts. I haven't thought of making them. They do make great gifts.