There are no "aftermarket headlights" available for the Z that I know of.
The nice headlights you see around are usually done by either yourself at home if you feel comfortable or by, for example, 180custom a company that does headlights - lot of the work you see around is theirs.
For something simple like Halos + headlight blackout, it will not even run you $400. Roll up to them in the house (brooklyn), and the job will be done on your fresh headlights and they even warranty it for defects for up to a year I believe... You're literally offering to pay the price of brand new for used. Definitely look into that if you're willing to drop 400 instead of trying to get used ones.
heres the link
Custom HEADLIGHT Paint & Lighting MODIFICATIONS|Photos & Details|UPDATED SUMMER 2011