The Subaru Splitters install right to the bumper of any 04-05 WRX . They're available pre-painted in any of the 04-05 STi colors.
Pricing is for a set of two splitter panels.
The splitters attach to a WRX STi using plastic body clips and automotive adhesive. The WRX bumper does not have the slots or holes for the tabs and clips on the splitters, and cutting them into your bumper is a little difficult, so you can also trim off the tabs, removing the fasteners, and using the adhesive or double sided tape for installation.