OER Commons is a large, multidisciplinary digital library of teaching and learning materials. It is searchable by both subject area and "grade level" (this includes post-secondary learning).

Provides access to curated online learning and support materials and content creation tools, led by an international community of educators, learners and researchers.

From the University of Michigan.

Simultaneously searches multiple repositories, could give you good ideas of other repositories to look in.

Searches content from over 60 different sources, you can sort by license type and type of resource.