Books on Board is a popular place to purchase epub or pdf books for you Sony.
If you live in the U.S., you can also try your local library to see if they offer ebooks for borrowing.
There are a few ebook search sites that will search based upon Author and/or Title and will give results from numerous web retailers
Addall
Ebookprices
Inkmesh
Lastly, B&N epubs can only be used on the Sony if the DRM is removed. We cannot instuct you how to perform this task but a Google search may help.