If you're looking for things to use from outside, or available from any garden center, you could try pea gravel.
Be careful though, "pea" gravel only refers to the size, not the composition of the stone. Any stones containing lime or other alkalines will fuck your pH.
Local "river rock" or "river gravel" is usually a safe bet, and available from many garden stores.
Whether you buy the gravel or find it in your backyard, use the following method...
FIRST, pour a bit of vinegar on the gravel. If it fizzes, you've got high alkalinity, and you don't want that stuff touching your plants. If it doesn't fizz, it should be fine. Just clean and sterilize with H2O2 or bleach first.
You could also splurge and buy a bag of hydroton. Its always a safe bet. And you'd be surprised how many uses you may find for it to use up the rest of the bag. Its always a good thing to have on hand.