If you’re looking for a Wii NAND backup file to download, it's important to know that NAND backups are unique to each individual console. Using a NAND backup from another Wii can permanently "brick" your system because it contains console-specific encryption keys.
What happens if you flash a stranger’s NAND to your real Wii? Your console will reject it. The system menu will check the keys, find a mismatch, and refuse to boot. You will have a brick. Even worse, you will have overwritten your own (possibly salvageable) corrupted NAND with a foreign one that will never work. There is no universal "stock Wii NAND" because the keys are the console’s identity. Wii Nand Backup Files Download
The standard method to generate this file is using BootMii. Here is the general process as outlined by the Wii Hacks Guide: If you’re looking for a Wii NAND backup
Step-by-step (using the most common method, LetterBomb): Your console will reject it
If you have to ask where to download a NAND backup, you are not at this skill level. This process requires soldering, hex editors, and deep cryptographic understanding.