Unsure what you mean here, to get door skins second hand is going to be interesting to say the least.
Your only options would be aftermarket or genuine from Subaru Parts Australia (genuine); or SSS Parts (aftermarket). You can take the skin off of an second hand door, but I doubt anyone will sell you one that isn't complete as the rest is useless.
In regards to quarter panels, quoka for a wreck would be your cheapest option where I'd assume they would say bring your own grinder and cutting disks.
Going through a wrecker would be another option for the quarter panels but you'd be paying more for what you'd be getting out of them.
Edit: LOL TAGS
