the Mfactory diff is Under $700 - http://www.teammfactory.com/helical-lsd_Subaru#
but ive heard mixed review about it. Everything else is either a clutch type diff or $1k+.
if you have your heart set on a front diff, get the JDM rallispec one, or the PPG front - http://www.ppgearbox.com.au/page.asp?productid=67
to be honest, it will be easier to find a used STI 6mt with a front diff already in it as part of a 6spd swap.
4-Time NNJR ESP Season Champion -15 ESP
3rd Place in 09' Pax Championship/Driver of the Year
2012 Northeast Divisional Champion in ESP