Before you search for that 460 MB NSP file, ask yourself: Do you already own the base game legitimately? If yes, updating via the eShop is effortless. If you are a pirate, at least verify file hashes, use a DNS blocker, and never run unknown executables.
Nintendo's legal team aggressively pursues the distribution of its IP. The recent legal action against the Yuzu emulator has altered the landscape. Consequently, many public "roms" sites have removed or restricted direct access to this title. Users often resort to private trackers or direct file hosts, though the stability of these links varies greatly. Mario vs. Donkey Kong Switch ROM NSP UPDATE