Low brass versions of the french horn method are now available in pdf format. They use the same exercises and melodies but are transposed to fit the trombone, euphonium and tuba beginner ranges. We don't have hard copies of these books but you can download them for free on the "low brass methods" page.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.