The Z28 spoiler and extension might be the following part numbers.
22946170 & 22946171
They are listed as 2014 only but currently discontinued. I've seen parts listed as discontinued before they have been available before.
Keep an eye on these.
1998 Pontiac Grand Prix - 11.24 @ 121
2009.5 G8 GT (Liquid Red)
GXP fascia conversion, GXP diffuser
2009 GXP M6 (MSM) #405 586rwhp/495tq
TVS2300, CTS-V Fuel System, Cam 222/230 116 lsa, Monster Lever 4 Clutch, American Racing 1 7/8 headers, rear Brembo's w/working Parking brake
11.91 @ 119