You can get one from a person in Castelia City; go to the first dock and talk to the first person. You will have a choice between a Fire Stone, Water Stone and Leaf Stone. It is really for the Pansage, Pansear, and Panpour but I just picked a random one since I didn't train mine up.
Another is in Desert Resort. Walk around until you see a psychic person, and I think it's a little north of him.
And finally, you can possibly find them in dustclouds in caves, including Wellspring Cave, Chargestone Cave, Mistralton Cave, Twist Mountain, Victory Road, Giant Chasm, and Challengers Cave. Unfourtunately there is not a very good chance of finding them. You will usually find a gem of some type
Hope this helps =)