You would have to find a rebuilder and see if they can get you what you need. Check with Subaru and see if they offer separate parts for your year model. There is a rebuilder here in Austin that I know can rebuild a Mitsubishi, so the parts are out there.
Though I did it to a Honda alternator but I was able to get a shim and the bearings from a dealership and the brushes from a local rebuilder. It wasn't that hard but I would recommend being patient. I put the bearings into the freezer for about a week and put the cases in the oven when it came time to put it all back together. I'd say go for it.