1 piece replacement rotors have been available aftermarket for many years. I assume they are still available.
As far as original 2 piece ones go, they haven't been available new for decades, other than lucking into or paying through the nose for NOS ones.
Original used ones can be cut on a rotor lathe if there is still enough material available to still have the minimum allowable rotor thickness left after cutting. That's about the only way to "rebuild" originals.