I also have a 2005 ZX3 S, which did not come with a rear sway bar. Steeda sells a kit that will allow you to bolt up their sway bar. I believe it's a 21mm bar as well.
Or, you could get the aluminum sway bar clamps that BiscuitNinja sells and use whatever rear bar you want. Make sure that the bar comes w/ the hardware to mount it though, otherwise you will have to get the endlink separately.
Personally, my setup is the BiscuitNinja aluminum clamps w/ the Saner/Auto-X ZTS 24mm adjustable rear swaybar. Total cost was around $250. Steeda's kit is about $150, but its not as beefy. Your choice.
BTW, this should be in the suspension section.