I bought one from California Baby. It is exactly like the Aura Cacia ones you can get online for a fraction of the cost.
I do the California Baby Cranky and OverTired EO Blend. I also use other EO's in these diffusers. You can buy replacement pads, so you don't have to reuse with different scents. I mean I reuse them, but I keep the peppermint scent pad with the peppermint EO. Does that make sense?