Pega Academy is a great resource for all Pega courses. Please visit https://academy.pega.com. Select Course Library>By Certification Path to find resource information related to System Architect Essentials I and II.
Pega is not open source like Java,Oracle etc. There are no specific materials for pega otherthan SAE 1(7.1) & SAE II(7.2) courses in the self study portal in pdn site. These courses are more than enough to understand and clear CSA.