A search would show up with the Ganzflow as the choice for most Impreza owners that are 2000+ they are at www.imprezars.com
Look for Larry Ganz or go to the product page.
I have a JCSports on mine and it's pretty cool, I lost some low end, but gained a decent amount of high end.
Cobb Tuning Has one
Weapon-R is cheap, but works.
There's a few, but it's 90 degrees and I can't think