You can find pond rocks at your local hardware store, pet store, and landscaping stores. If you're looking to get rocks of any shapes or sizes, then you can look almost anywhere. Construction sites, around your neighborhood, near the creek.
If you're still unsure and would rather not waste too much time running around searching for certain size and shape rocks and stones, I'd recommend heading to your nearest pet/fish store or hardware store such as HomeDepot, and see what they have in store or whether they know of any local supplies. They may know of some local areas that the general public would not be aware of.
