Musical U: This website is headed by a great music educator named Christopher Sutton. It's an outstanding resource for all things related to Music Theory, including ear training, scales, chord building, the Circle of Fifths, and more! It's subscription based, but there's tons of free resources!
Recommended Baritone Ukulele Resources
Humble Baritonics: Jeff West over at Humble Baritonics has created a one-stop shop for all things related to the baritone ukulele. You can find blog posts, lessons, news, festivals, and much more there! Definitely worth having on your radar!