I know you are likely looking for used but if not you can get them on amazon now that’s not an item people get rid of often.
I found it on amazon India. It is actually a case meant for carrying headsets/pendrives/cables etc. but the size and shape makes it a perfect snug fit for a gameboy.
TIZUM Travel Organizer Case for Earphones, Pen Drives, Memory Card, Data Cable (Black): Amazon.in: Computers & Accessories
get like 2-3 hours with these playing with my ever drive
https://www.amazon.com/Energizer-Lithium-Batteries-Battery-Ultimate/dp/B00003IEME
rechargeable are more like 2 hours but if you have multiple sets its not really an issue