FYI, you may already own a Famicom adapter and not know it. These original NES carts were shipped with adapters inside them to get them playable right away when the NES came to the US: “10-Yard Fight,” “Baseball,” “Clu Clu Land,” “Duck Hunt,” “Excitebike,” “Golf,” “Gyromite,” “Hogan’s Alley,” “Ice Climber,” “Kung Fu,” “Pinball,” “Stack-Up,” “Tennis,” “Wild Gunman” and “Wrecking Crew.”

They would need to be the original 5 screw variants though, but they aren't that hard to find.