You still get runecloth from outland mobs, so go level. You'll also pick up netherweave(which you need absolutely LOADS of), so at least you'll always be picking up cloth that will help sooner than later, and getting your levels. Go LFG for ramparts or blood furnace whenever you can too, and you'll pick lots up.
If you really hellbent on runecloth though, western plaguelands is definitely the place.
Also....
Quote:
You'll also pick up Argent Dawn rep, which is good for tailors as they've got a couple of tailoring patterns.
... this is true as well.