Retrofitting, as in replacing the regular non-HID housing with the HID housing.

Anyway, if ya want the housings: http://www.toyotapartsales.com/

Under "Mechanical Parts Catalog" select either year 2003 or 2004. Either one works. Click search button.

Next page, select "celica gt-s" as model, it should take you forward.

On the menu on the side, select "electrical" > "chassis electrical" > "headlamps" > "composite headlamp"

What you want is "COMPOSITE HEADLAMP, Celica, w/HID" which are $231.45 each.