I am assuming you have an automatic transmission, because the manual transaxle does not have a filter. It is located under the transmission pan and should cost about $40 plus a new gasket and labor if you have someone do it for you.
The 2006 Subaru Impreza has a 5-speed manual.
The 2004 Subaru Impreza has a 5-speed manual.
The 2008 Subaru Impreza has a 5-speed manual.
The 2014 Subaru Impreza has a 5-speed manual.
The 2012 Subaru Impreza has a 5-speed manual.
The 2002 Subaru Impreza has a 5-speed manual.
The 2010 Subaru Impreza has a 5-speed manual.
The 2009 Subaru Impreza has a 5-speed manual.
The 2001 Subaru Impreza has a 5-speed manual.
The 2007 Subaru Impreza has a 5-speed manual.
The 2013 Subaru Impreza has a 5-speed manual.
The 2011 Subaru Impreza has a 5-speed manual.