http://www.tirerack.com/suspension/S...6&autoModClar=
https://www.ecstuning.com/BMW-Z4-M_R...n/Shocks/Rear/
Did a quick search of tire rack and ECS tuning which are 2 suppliers this side of the world.
They do not list Konis for your car but obviously are available.
The Bilsteins are decent OEM replacements.
http://www.turnermotorsport.com/BMW-...ks-struts.aspx
Edit:
Koni does not make struts or shocks according to their website.
Also the ad you linked to is for one shock only.