You'll want to find benign rocks, rocks that won't affect your water parameters. For the most part, you're generally safe, and there's a quick and easy test you can do with vinegar to see if what you've found is safe. The best place to find rocks, IMO, is at a local landscaping company. Stay away from limestone and similiar stuff that will leach into the water, otherwise you're pretty much okay.
I'd recommend that you clean them thoroughly and boil them before putting them in your tank. Yes, boil them, as in put them in a pot on your stove. Funny stuff, right? This will ensure that there's no living nasties on them that you'd want to prevent getting into your tank.
