Sorry if I'm being completely clueless here but what kind of "cartridge" are you using in your top photo?

https://s3.postimg.org/nb0cithyb/image.png I saw these on a YouTube video for an sp inside a DMG. They look good imo, almost as though they belong to the game boy! I'm looking for a shoulder button solution for my project and this is by far the nicest looking and most practical ones I've seen in an...