For an ebook version, search for the title in:

• HathiTrust - here's their Accessible Text Request Service page.
• Internet Archive try their BookReader tool.  It's the headphones icon near lower right corner of the individual book record window.  For in-copyright books, borrow first, then the icon will appear. This is their Books for People with Print Disabilities policy.
• Bookshare - A collection of over 1.3 million titles.  Including textbooks, bestsellers, children's books, career resources, and more. 
• EMMA - Educational Materials Made Accessible 

Email John Laskowski at jdlasko@illinois.edu for assistance.

For an audiobook version of a book, check:

• National Library Service for the Blind and Print Disabled - Braille and audio materials (books, magazines).
• Bookshare - A collection of over 1.3 million titles.  Search for, or request, an audiobook version of in-copyright books. Includes textbooks, bestsellers, children's books, career resources, and more.  They offer ebooks in audio, audio + highlighted text, braille, large font, and other formats.
• Learning Ally Audiobooks for Dyslexia & Learning Disabilities

For a digital Braille version check:

• National Library Service for the Blind and Print Disabled - Braille and audio materials (books, magazines).
• Bookshare - A collection of over 1.3 million titles.  Search for, or request, an audiobook version of in-copyright books. Includes textbooks, bestsellers, children's books, career resources, and more.  They offer ebooks in audio, audio + highlighted text, braille, large font, and other formats.