History Center of San Luis Obispo
The San Luis Obispo History Center holds the George Loorz Papers, building contractor at Hearst Castle. Contact the History Center to learn more about their collections: https://historycenterslo.org/about/administration/
UC Berkeley
The Bancroft Library and Environmental Design Archives at Berkeley hold major collections of Morgan materials. Find them on the Online Archive of California.
Hearst Castle
San Simeon State Parks manages the historic records at Hearst Castle and gives additional archives recommendations for Hearst and San Simeon research.
All California Archives at the Online Archive of California
Here is a link to search many California Archives that hold materials related to Julia Morgan.