In our experience, you do not need to replace your pistons which is why we don't carry them in our inventory. These brake pistons are composed of chrome titanium steel alloy which is extremely strong and hard and will not wear or corrode. Whatever surface discoloration or apparent pitting can be polished out with number 1200 emery paper and is certainly no reason for replacement.
A jammed or locked piston also does not need to be replaced. Your piston has locked up because the seals are bad causing brake fluid and debris to blow into the piston chamber and gradually coat the walls with residues. You should take special care to clean and polish the chamber walls then install new seals which come with most brake caliper rebuild kits.