As above really;
Free Public domain in various formats:
Mobile read books
Gutenberg
Feedbooks
Smashwords
Sribd
For purchased stuff:
Amazon - needs kindle app
WHsmith
Waterstones
Ibookstore via the ipad itself of course
Worth bearing in mind the great software in the forums here if you haven't already,
Calibre for library organizing and converting formats - and
Sigil for modifying, fixing and adding covers for epubs.