I'm running Sony ebook Store 3.1 (Sony's ebook software). When I click on the ebook store, it takes me to the ebook store homepage which has a section on the right that says "Unearth a classic." Once I log in, there is a bunch of ebooks that pop up. The description says "Choose from over 500,00 free public domain titles." I think that's what you are looking for?
Also, on the bottom of the homepage, it shows a Library Finder. I believe that allows you to find libraries in your area from which you can borrow ebooks from if you have a library card with them.