I agree about the boiling. I have gotten rocks from nature before that have worked beautifully, but boil them for several minutes (I do 15-20 because I'm paranoid). If you're worried about the rocks causing harm to the pot you are boiling them in, you can put a washcloth underneath them so that they don't cause scrapes or other damage to the pot.
If you want to buy cheap rocks, you could check out flower stores too for bags of relatively cheap smooth stones (normally used to put at the bottom of pots to help with water drainage). You can probably even go to the craft/garden section at Walmart or Target to find cheap decorative rocks.
Good luck!