Q:
Is it possible to get order-specific armour from other orders?
Not sure myself, but if you check out the PvP locker in Heart of the Mists, you will find that there is a storage spot for each order set, so the pieces are most likely available at higher PvP levels. Again, not sure if you can use transmutation stones to transfer the appearance to PvE items.
You can test it out by using a level 80 trans stone on a pvp piece and try keeping the appearance of that item.
I was of course referring to PvE. I was using the PvP locker to pick out a look I liked actually, but it involved a piece from another order; which may be impossible to get. Hence my question.
Oh, and it turns out transmutation stones cannot be used in sPvP. Not really surprised.