Bookshare is a digital library geared towards providing accessible literature to students with disabilities that impact their ability to read. This resource is free for qualifying students, as described on their about pages.
CAST is an organization focused on increasing equality in learning. One major area that their resources focus on is the Universal Design for Learning, which is an approach that emphasizes taking a stance to meet diverse needs of students. Their website includes dozens of free resources to implement a UDL approach in the classroom.
DO-IT is a resource for both those with disabilities and those seeking to be allies to the community. Their resource pages include webinars, events, scholarships, books & training materials, and links to other resouces for folks with disabilities and those learning about disabilities.