Your back yard, local rivers, streams, and lakes.
Just boil it if you're worried about anything. The worst I've ever had is some white fungus seep out, but in time and some scrubbing later, even that disappears.
Collecting your own not only saves you plenty of cash, but looks much more natural, IMO. It's also easier to get those nice big pieces that can really fill a tank, where you'd be spending an arm and a leg at the LFS for such a piece--if you could even find a big enough size. Otherwise you just have a jumble of similar-sized pieces, which I think looks kind of funny.