Mises Institute is probably the best source for Rothbard's work. They might have a lot of unpublished audio recordings of his lectures. Ngl, simply reading everything he has published would itself take a lot of time 😂

The Mises Institute Alabama has the entire personal library of Rothbard. I was browsing it for hours, and the highlights were his handwritten notes inside Das Kapital, both hilarious and insightful.