Door not staying open

The drivers car doesn't stay in the fully open position. The Haynes manual doesn't say much about doors, and I haven't found anything from searching the forum.

Looking at it, I think the piece that is bolted to the door pillar and has a silvery bar that goes into the edge of the door is the problem. On the passenger door this can't be moved except by moving the door, but on the driver's side it is slightly loose at the door end. Anyone know whether this is generally something that is fixable within the door, or it is easier / cheaper / quicker to get a replacement part?