Maybe they've been there for a while, but I don't remember seeing them. Individually for $24.99 in HD or $4.99 to rent, or available as a bundle of the three movies for $44.99.
You can also buy all three books in digital form for only $17.99.
I've read the series three times, watched it twice, but still tempted to pick up digital copies. Once our kids destroy the DVDs we have, I'll come back for the videos.