K&N 57-0626 Cold air intake kit suits SUBARU WRX & STi GDB GDA 00/07.

It is also reported to fit 98-00 GC8 WRX & 2000 Forester but is not listed by K&N to do so - so  cannot guarantee.

Part 57-0626 Product Specifications
Product Style: 57i Series International Kits
Type: Gen I
Street Legal In All US States: No
TUV Approved (Europe): No
Clamp Style: Regular
Replacement Filter: RC-70031
Air filter inlet flange: 98mm
Weight: 2.9 lb (1.3 kg)

Product Box Length: 9 in (229 mm)
Product Box Width: 9 in (229 mm)
Product Box Height: 9 in (229 mm)

Fits the following Subaru Impreza, WRX, STi
2007 SUBARU IMPREZA WRX STI 2.5L H4 F/I - All
2007 SUBARU IMPREZA WRX STI 2.0L H4 F/I - All
2007 SUBARU IMPREZA WRX 2.5L H4 F/I - All
2007 SUBARU IMPREZA WRX 2.0L H4 F/I - All
2006 SUBARU IMPREZA WRX STI 2.5L H4 F/I - All
2006 SUBARU IMPREZA WRX STI 2.0L H4 F/I - All
2006 SUBARU IMPREZA WRX 2.5L H4 F/I - All
2006 SUBARU IMPREZA WRX 2.0L H4 F/I - All
2005 SUBARU IMPREZA WRX STI 2.5L H4 F/I - All
2005 SUBARU IMPREZA WRX STI 2.0L H4 F/I - All
2005 SUBARU IMPREZA WRX 2.0L H4 F/I - All
2004 SUBARU IMPREZA WRX STI 2.0L H4 F/I - All
2004 SUBARU IMPREZA WRX 2.0L H4 F/I - All
2003 SUBARU IMPREZA WRX STI 2.0L H4 F/I - All
2003 SUBARU IMPREZA WRX 2.0L H4 F/I - All
2002 SUBARU IMPREZA WRX STI 2.0L H4 F/I - All
2002 SUBARU IMPREZA WRX 2.0L H4 F/I - All
2001 SUBARU IMPREZA WRX 2.0L H4 F/I - All
2000 SUBARU IMPREZA WRX 2.0L H4 F/I - From 10/00

Installation Instruction Sheet