I actually ran into the same problem. I didn't have a good super local places to buy any. I ended up buying a box of 25 3in - 4in in diameter smooth flat rock river rocks off of Etsy.
They were listed as all natural right from a nearby creek. It was also noted that they were not cleaned with any chemicals.
Was something like $8.95, plus $13.95 shipping, bit on the pricey side but it ended up being the most convenient for me. Just make sure to scrub them well and all that. If I can remember correctly I had also seen some stuff on EBay.
I've you're interested PM me and I'll give you a link to the Etsy shop I found mine on. They had all sorts of sizes, shapes, etc.. =)