Talking to Free Play, asking them about how they produced the gameboy carts they have on display:

• They use GB Studio to make the games
• An Australian company called Inside Gadgets sell the PCBs for Gameboy carts
• Recommend using the GBxCart RW for flashing
• Also suggested flashing to bootlegged carts instead of buying new pcbs, if buying new PCBs is too expensive.
• They crowdfund their production runs on Kickstarter