There’s a bunch out there.
Check out eBay for the stuff of a bodymans nightmares.
The best looking one that I have seen is RPMs Techniks version. They make a whole body kit that goes with it too. (There is one on the front cover of issue 295 of 911 & Porsche World).
https://rpmtechnik.co.uk
https://csrporsche.co.uk/models/996-models/
I think Getty may make one or two of them as well.
http://www.gettydesign.com/996tailbases/street-wings-tailbases-and-spoilers/#ducktail-classic
I’m a bit of a Getty fan boy...
There is also a company - I think out of Holland, that make a whole modern version backdate kit which is quite nice if you are into that sort of thing. There is a duck tail with that too.
http://forums.pelicanparts.com/porsche-911-technical-forum/845692-back-dated-997-a.html
That’s based on the 997 though.