Meremark, I don't know whether you have done this, but compare the prices on the Sony store to Amazon. They are often the same. Amazon used to be less expensive, but now, especially with a Sony, it pays to shop around.
Besides the stores already mentioned, don't forget your library!
Most library books are available in epub format (as you may already know).
But yes, if you don't mind stripping the DRM, conversion in Calibre is easy (once you have an uncopyprotected book).