Doubt a wrecker would sell you the mirror as a standalone 'unless the rest of the side view is cactus'.
Try e-bay as mentioned above, you can also get these made up at places such as Windscreen O'briens etc 'sounds like the e-bay option is the cheapest though'.
Unsure if you can purchase these as a seperate line item from Subaru.
As always, before laying cash down to a windscreen place/wreckers etc make sure you cross check what it would cost to either buy the part 'if possible', or the unit brand new as sometimes you'll be horribly suprised.
Another thing to consider 'this may have changed though', I don't think you'll be able to get the backing part of the mirror 'the rubber surround', from a windscreen place unless they're able to utilise the existing one.
If you can source the e-bay option for anywhere near the price mentioned above, I'd be buying quite a few spares.
