Thanks!
There was only one cartridge ever made and it was a gift to someone, so there aren't any available for sale. You can buy a blank cart from a third party and flash it with the downloadable ROM for yourself though.
It uses MBC1, ROM banks 0,1,2,3 and no RAM banks.
Here is what I used. They can flash the ROM for you if you email it to them. Any of the MBC1 firmwares should be fine, no battery required.
https://shop.insidegadgets.com/product/gb-2mb-128kb-sram-flash-cart/
If you do that, I can share cartridge label graphics.