Sarah, another source for eBooks are public libraries. I've borrowed books in ADE from my local library. They're free and you get to keep them on your reader for 21 days at the end of which they disappear.
In the UK, the Essex public library offers eBooks on loan here (
http://essex.bookaisle.com/CBEBC5EA-...ookFiction.htm)
Some US libraries allow non residents to borrow eBooks for a small price. Check out the Wiki on Libraries here
https://wiki.mobileread.com/wiki/EBook_Lending_Libraries