Challenges of English for specific purposes (ESP) course development and instruction have still been reported in a number of studies. They could affect ESP teachers' belief and confidence in their ability to do such tasks. This qualitative case study explores the development of self-efficacy, along with source information concerning challenges in course development and instruction. The study case took place in a Thai university's institution where eight ESP teachers participated in this exploration. Through multiple interview sessions, they revealed source information and different ways to develop self-efficacy. Findings included that cognitive and enactive mastery experience were the most influential sources derived from their educational background and success in implementing pedagogical elements in their actual practices. Vicarious experience and verbal persuasion appeared to be scant due to the underlying contextual factors related to the institutional policies explored in the case. Physiological and emotional states proved to be less influential and acted as supplementary sources of influence. The current study also highlights the influence of contextual factors' shaping sources of self-efficacy and the development. Indeed, the lack of collaborative practices diminished vicarious experience and verbal persuasion so that individual teachers relied on their knowledge and experience as mastery experience to be efficacious. Research implications focus on policies where collaborative culture development is included. A suggested plan is to employ professional learning communities (PLC) to develop internal collaboration (e.g., sharing knowledge and practices to improve self-efficacy sources and development).