Cadence of Hyrule has a release date of 13th June 2019. This date was announced during the Cadence of Hyrule release trailer at E3 during the Nintendo Direct video.
Upon release, the game will only be available in digital format directly from the Nintendo Switch shop. There is no physical release at this stage.
Update: Cadence of Hyrule is now available on the Nintendo Switch Eshop!