Manly Palmer Hall is the best source I've managed to find on this sort of subject.

https://www.youtube.com/user/PromienieNet/playlists?sort=dd&view=50&shelf_id=3

This channel has some great lectures by him plus some other interesting things if you click "playlists" The link to Mark Passio in the firs post is another great source and more current. Still Manly P Hall is my all time favorite, I listen to his lectures a lot when I browse the internet and when I go to sleep I play it in the background.