If you need to convert or split the file (for example, if it's over 4GB and your drive is FAT32), tools like Wii Backup Manager are the standard for managing these libraries.

This guide does not provide links or endorse piracy.

In most regions, downloading a game you do not physically own is considered piracy. The legally correct way to obtain a WBFS file is to rip it yourself from your own personal copy of the Super Mario Galaxy 2 disc. Tools like CleanRip on the Wii or simply using Dolphin's "Dump Disc" feature can create your own WBFS file from your original disc.