While Dolphin cannot legally "download" a copyrighted Wii NAND from the internet for you, it includes a built-in feature to perform an Online System Update
After importing:
There are two primary ways to obtain a NAND. One is highly recommended for legality and functionality, while the other is a built-in fallback. Method 1: Dumping Your Own Wii NAND (Recommended) wii nand download dolphin