I picked up 4 new stock pistons from one of our better known engine builders for $50 shipped. Rings were another 120 ( have to buy a set of 4) . The cylinder was in excellent shape so I just replaced the one that was bad. About 6 months later another one went so I decided it was time to go with better pistons.
I was pushing 400whp which I believe to be the limit of the stock sti pistons in a high compression (9:1) hybrid setup. If your power is near the limit of the pistons I would recommend upgrading so you don't go through what I did.