Subaru stocks 0.010" and 0.020" standard overbore pistons (or at least they do for the EJ22). They were really cheap for my car too, only $45 or so apiece.
I doubt you'll find much of an aftermarket that would make 0.040" over pistons and be sure they have the same or greater weight as stock, but STis do get a lot more aftermarket attention. At the very least, they won't be as good a value as the Subaru ones.
-N