Pretty good place to get the gaskets/head bolts/timing belt components:
https://sixstarbernie.com/
I've been watching for this same leak on my 05 Legacy wagon with 115k for a while. My friend that has an 07 Outback had his begin leaking profusely at 106k. It was pouring oil out like a sieve and burning off the exhaust. Went from dry as a bone one day to pouring oil the next.
What I've noticed: The OEM "blue" Subaru filters for these single cam engines suck. The gasket is very small and not very tall. The last one I hand installed, the top of the filter canister bottomed out on the engine block. That's no good so I took it off and tossed it. The cheap Purolator filters aren't much better with regard to same, although they do have a much wider and flat gasket as compared to the "O" ring shape of the OEM filters. The Wix filter 57712 seems to be the pick of the litter. I also think I've been lucky by using 10W-30 instead of 5W-30. Mobil 1 5W-30 seems thinner than water.
Bob, before I went down the long road of repairs on what you've depicted, I'd try an oil change with a Wix filter and 10W-30 oil, clean up the bottom of the engine and monitor for a few weeks. You might even try to slip the thinnest of feeler gauges between the existing filter and the bottom of the engine to see if the last person that changed the oil didn't "bottom out" the filter on install, or worse "double gasket'ed" it.