I used a custom white silkscreened 128KB insidegadgets flashcart with rumble option. He sells a 4MB version but it will be green and won't have some of the branding mine has https://shop.insidegadgets.com/product/gameboy-4mb-32kb-fram-flash-cart-ultra-lo...
Alternatively I have very few remaining boxed copies left that I wouldn't mind selling one you want to reach out on the Discord (found on the Kickstarter page).