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Any place that has free, DRM'less book books can be put on a Sony. Just convert the DRMless copy to epub.
But you can do a search at Inkmesh just for Sony books. Just be aware that sometimes their info is outdated and there's only a few right now but it might be worth bookmarking. Inkmesh
Baen has a large Free Library for Scifi and Fantasy. Here.
Smashwords has all kinds of Freebies. Here.
Check out any ebook publisher who's not Amazon and see what formats they offer, and if they have a freebie section.
I found a number of Theresa Weir/Anne Frasier stories that were unlimited. Happy dance. But mainly I look at Books on the Knob. Many of the free books are offered at ARe or Kobo. (And they list the Kobo codes there.)
I read SciFi.
Baen (google your way there) publishes a lot of SciFi, and they have ebooks without DRM at bargain prices (just released ebook $6.00!) also lots of titles in teh free library, including many you'd not expect to be there.
Several formats are available. Sony uses epub, like the Nook.