From looking at the parts diagrams (see:
https://www.subarupartsdepot.com/par...iagram=7610075), it seems as though Subaru does not sell the o-rings, bolts, and other ancillary components for the AC system independent of larger component groups. Unless someone else chimes in with experience working on Subaru HVAC systems, I would imagine your best bet would be to head to either your local parts store, or the Subaru dealer, and see if they stock the individual components. Perhaps removing the discharge tube assembly entirely would help clarify your situation, especially to a local parts store clerk?
My limited experience with AC systems is that auto parts stores often supply a complete gasket rebuild kit for the whole system. You may be stuck purchasing this in the place of an isolated gasket.