On that Raleigh, I believe it's a plastic doodad that bolts to the underside of the frame. They're going to be difficult to find, so if it were me, I'd hit the local bike shops for a metal cable-housing stop that clamps around the frame tube. Something like this one, except with housing stops on
both sides:
Replace the plastic doodad with the housing stop. Mount the housing stop so that the fitting from the old plastic doodad prevents the housing stop from sliding down the downtube.