I finally found an American-style op shop in the inner west. It looks like they rotate the oldest things to be half price, and there's a $2 rack. Things still aren't by size (WTF Australia?) but at least there's a fat girl rack so I don't have to scavenge through a million size sixes.
I ended up getting 4 men's t-shirts for $9.50, so there will be new underpants in my future.
Marrickville
Clothing & accessories147 Marrickville Road
Phone: (02) 9572 6065
Opening hours:
Mon to Fri: 9.00am to 4.45pm
Saturday: 9.00am to 1.45pm