You have to buy the whole switch , that's part of the switch , You could buy a switch and remove the plastic button from that and fit it onto your switch body , but that's a little pointless.
At the scrap yards this isn't an expensive part , I think at most it should cost you between R50 and R100 a switch for the single switch at most , If this is the front center console switch then it may be a little more . I had to replace mine on the left rear door and it cost me R75 .
You cant buy the piece separately , as far as I know .