If there's a pond or lake your city draws water from, it;s probably protected from people dumping anything into it... so it you want to be sure of finding a clean piece of driftwood, search where you know there is clean water. Look it streams where water doesn't get the chance to become stagnant, and you'll find some great driftwood ;-)
Good luck, and make sure you give it a decent soak in some chlorinated water for a couple of days before adding it to your tank. By the time you go to take it out the chlorine will have evaporated, although you might want to add a little dechlorinator just to make sure you've got all the chloromine too.
