http://www.cushieaudio.com.au/brand/gro ... -usb3.html This is what you want if you want USB/aux in.
Otherwise you need to buy a "subaru AV panel set" to suit liberty/legacy/outback. You can find these on ebay or on JDM sites, but they run about $300. On top of that you need a stereo, be it single din + pocket or a double din.
Grom = cheaper but stock headunit
AV panel + headunit = expensive but more potential for aftermarket sound system
I have currently gone with the Grom in my outback and am just having a small issue with the firmware on it where it does not recognise the unit when turning the car on sometimes, which Grom are kindly writing new firmware to fix the issue. I got mine through Cushie audio and the tech support from Cushie and Grom have been outstanding (Cushie contacts Grom for you).
The grom is an excellent unit and produces pretty good sound through my stock Kenwood system, however if I had money to burn I would go the AV panel, and aftermarket headunit route purely for the ease of upgrading the sound system later on.