In this mixed-methods study of large enrollment introductory physics service courses, I investigated students' perception and use of online education resources as supplements to course-provided materials and activities. Specifically, I focused on the increasing use of popular free online media resources such as YouTube and Khan Academy, and fee-based textbook solution repository services such as Chegg. In the quantitative portion of this study, I surveyed students from three courses on their textbook and online resource usage and found that most students relied primarily on online resources as they navigated the courses, and comparatively few used the textbook regularly. In the qualitative portion, I investigated the patterns and culture of textbook and online resource usage via semistructured interviews and found that students reported using online resources as supplements to, or in place of, the course-provided materials when engaging with online homework or studying for an exam. Students reported using online resources productively to guide learning efforts, but also acknowledged unproductive uses such as copying solutions to mitigate loss of assignment points. I provide suggestions for changes in course materials, practices, and expectations to better engage students in the course and in their learning.