I'm getting the E-books (free) from a few places online.
This one is not free but offers some great books for those looking to add some others to RC: http://www.yesterdaysclassics.com/
If you have different types of Ereaders and need the Ebooks in differing formats, for this I'd recommend a free program for your computer called "Calibre"
With this program, after you download a book from one of the sites above you can then have Calibre "get books" from your computer and then uploaded the books into the program. This is a great feature because you can download any format at all and the Calibre program will convert it to your specific E-reader. So for example most of the google books that are free are in Epub, but Kindle for example does not read that format. So Calibre converts the books so they can be opened in Kindle and read just as any of the other books. Also any books already on your E-reader can be added to the Calibre library.
Hope this is helpful to some.