There's a couple of options. There are pull chain switches you could install into a flush mount light. It would require some modification (drill hole, etc.) of the fixture and wired properly so, it would have to be done by someone who knows what they are doing. There are also flush lights that come with a turn knob switch installed. You can swap that for a pull switch. If you want to do away with the string or chain, why not add a remote unit in the fixture and mount the remote control unit on the wall nearby? The big box stores and local hardware stores stock them You could also install one of those types that Billy Mays used to advertise.