any autozone type store. just ask for an 04 or 03. they dont have 05 in their computers yet.

yup. just did oil change on my 05 wrx and used an "03-04" oil filter

SUBARU OE OIL FILTER: 15208AA080
PUROLATOR OIL FILTER: L14612
FRAM OIL FILTER: PH6607
WIX OIL FILTER: 51365
K&N OIL FILTER: HP-1008
AMSOIL OIL FILTER: SDF13
AC DELCO OIL FILTER: PF1237

From here:

http://www.scoobymods.com/forums/showthread.php?t=3014

I dont have a wrx yet.... but on my current car I switch from cheapo fram to k&n filters. There was a huge difference in how clean the oil was with the k&n compared to the fram. K&N filters are suposidly just mobil 1 filters relabeled.

I just buy the stock filters from the dealer. $11.