The problem has been found and fixed! It turns out that the sway bar on that side is just slightly bent (probably from an impact before we owned the car - we bought it used), and because of wear on the right ball joint, the sway bar was sometimes coming into contact with the piece below it when we turned. Our mechanic replaced the right ball joint this morning, and voila - no more clunk! Incidentally, it was actually a body shop guy who located the problem - our mechanic advised us to see one this morning - and then our mechanic fixed it. Thanks so much for all your help everyone - we were able to discuss this whole thing much more knowledgeably because of you, and I know that helped us reach the solution!