Are you looking for ebooks as in: I want to buy some ebooks?
Or as in: I have this sh*tload of paper books that I would like to have as ebooks because I don't want to strain my back carrying them around?

The former is rather easy to solve and has been pointed at above; the latter requires some research and a bit of willpower and restraint. Apparently (I would of course not know) the P2P networks are full of entire libraries of ebooks; one could argue that if one already owns a paper copy of a book, obtaining an electronic version of same would be quite ethical.

The willpower and restraint lies in that one would only keep those torrented ebooks that one indeed already owns.

Naturally: allegedly, apparently and according to rumour and popular lore...