I have just recently ordered this jounce bumper (25993737). It came with the foam spacer and the rubber "ball" as an integral part. Colour of the plastic was kind of purple.

 

Ok.
Part #25993737 is for the regular HHR's (LS, 1LT, 2LT )
......#19352221 is for the HHR SS.
Maybe the SS one has a shorter center bumper ? Not sure on this.

You can't buy the center rubber bumper part separate since it is pressed into the jounce assembly.