I go to ManyBooks.net, Project Gutenberg, or Google Books and pick up free .EPUB books by the dozens for mine. Today I uploaded the original Tarzans, Barsoom, and Allan Quartermain books. (My tastes run from classics to pulp and back, I'm in a pulp phase now I guess.)
I also pick up free indie books from LiveJournal, WattPad and other sites and convert them to PDF or EPUB for my Nook.