the thing about shimming is that it will effect the handling of the car. I work on Army helicopter engines and there is a close tolerance to parts. Now you can't compare oranges and apples but you get the idea. I tried car-parts but all they have is Rear Crossmember.
Not online, but just go hunting junk yards. I have found 4 or 5 turbo legacy crossmembers in the last year or so. Where I got mine paid like $30 for it, easy to pull since the engines are usually gone quick from people using them in sandrails.