Downloading a NAND dump from the internet for a physical Wii is because every console has unique encryption keys fused into its hardware. A NAND backup from one Wii will not work on another without specialized tools to modify the keys. Instead, you should create a backup of your own console's NAND for brick protection or for use in the Dolphin emulator. Creating Your Own Wii NAND Backup
These links generally lead to software that pulls official system files (NAND data) directly from Nintendo’s servers (NUS).
If you search for a pre-made NAND dump online, you will find results. However, downloading a NAND backup that was created by someone else is highly discouraged for several reasons: