i don't even know that the dealer has a part number for this, but you can look it up on Subaru's opposed forces website or call. they probably just use a rebuild kit or replace the power steering pump unfortunately.
This oring size will work if you want to order one from online or a local store:
1/2'' ID, 5/8" OD, 1/16" Thick
This oring size will work if you want to order one from online or a local store:
1/2'' ID, 5/8" OD, 1/16" Thick