I thought 5/8 went out years ago. Might have better luck finding adapters from 1/2-5/8 or 5/8-3/4. I have been trying to complete a 5/8 drive set of impacts myself with no luck.
There were a lot of Industrial manufacturers (Apex-CP-IR-Roto-Proto) that made these back in the 70's & 80's for assembly line use! Maybe try industrial auction sites or government surplus sites??