1) YOU DON'T! Wii Menu v1.0 may have been a launch-day only version & may have been replaced with Wii Menu v2.0 immediately for region specific settings, SD card support & downloadable updates.
Therefore, it'll be extremely difficult or impossible to find it.
2) Why do you really want it? No, Seriously! WHAT DO YOU REALLY WANT IT? -- Attempting to downgrade a Wii is UNBELIEVABLY, INCREDIBLY, EXTREMELY FOOLISH to attempt as it's typically the most guaranteed way to permanently brick your Wii.
You can practically do anything you wanted in Wii Menu 4.3 (which is the final version of the Wii Menu) for modding purposes (as all the good mods have been updated to support it), making downgrading pointless.