The only legitimate way to obtain the game files is by owning a physical or digital copy of the Wii U version and "dumping" it from your console.
If you already own a Switch, why go through the trouble of emulating the older Wii U version? The answer lies in raw performance and customization.
So, why do gamers opt for a Cemu ROM of Zelda Breath of the Wild? Here are a few reasons: