YouTube is your friend.
And this user is also your friend because he/she shares with you the knowledge of knowlegde for singers.
Here you will find ca. 300 videos of masterclass (only with sound, no image) recorded in 1971-72, at the Julliard School (where the ballerinas are really running around in tu-tu skirts and stockings... I saw that with my own eyes!!!)...
And of course Maria is giving those incredibly useful classes about how to sing. And yes, she is cruel. Ok, not evil cruel but supporting cruel, because she knows everything so you should listen to her!!!!!!!!
Enjoy, and use the force!!!!!