The most "legal" way to acquire Switch ROMs is to "dump" them from your own hardware. This involves: Owning a hackable "V1" Nintendo Switch (unpatched). Installing Custom Firmware (CFW) like Atmosphere.
Most ROM repositories are filled with aggressive pop-ups. Verify File Extensions: Stick to .NSP and .XCI. The Ethical Middle Ground: Dumping Your Own Games nintendo switch roms free