Final answer:
Letters from early church leaders are found in the New Testament epistles, which discuss various issues and help establish orthodox beliefs and practices.
Step-by-step explanation:
Letters from early church leaders that tell about issues the early Church had to face are contained within the New Testament of the Christian Bible. Specifically, these texts are known as the epistles, which include letters from St. Paul and other apostles who addressed various doctrinal disputes, moral guidance, and group organization during the formative years of Christianity. The process of establishing a canonical set of texts was marked by debates and arguments amongst early Christians, with the essential objective of agreeing upon orthodox belief and practice.