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Searching for a "Nintendo DS Menu ROM download" will yield many results, but the only ethically and legally defensible method is dumping your own console.
This write-up is for educational and preservation purposes only. All trademarks are property of their respective owners.
Unlike a standard game ROM, the Menu ROM contains — but it does contain the system’s BIOS-like interface, the “Health & Safety” screen, and the cartridge-slot detection routine.
Unlike game ROMs, the owned by Nintendo. Distributing or downloading it from public websites violates copyright law in most countries. Emulators like melonDS explicitly require you to provide your own BIOS/firmware dumps from a console you own.