Rock painting doesn't require a lot of supplies to get started. A few paint colors, brushes and rocks will get you going.
Depending on where you live, you might have a large supply of nice smooth rocks near your home. I know many rock painters that find rocks in riverbeds or around lakes around where they live.
Some dirt fields are full of rocks that farmers would love for you to take, but ask first. You might find them walking or driving down the road.
Keep your eyes open for rocks and you will start seeing them every where. Ask your family and friends to start looking for them for you. My sister often brings me back a few rocks from different countries she visits.
My adult son has gone rock hunting with me and helped carry them back to the truck. My adult daughter has collected rocks and carried them in a backpack all day, while on vacation. To me, I enjoy getting a bag of rocks as much as flowers, especially when they come from people I love. Of course they know I'm a little rock crazy, not every woman would rather have rocks than flowers!