I'd say to try the search engines. The interface is annoying at some web sites, but ultimately, because you have a Sony instead of the Kindle or Nook, you can pretty much shop anywhere, so try
www.inkmesh.com. You can browse by subject and you can search by title, author or keyword. It will show you everywhere the best prices are. Just ignore results from Amazon or B&N (since they will not work with your reader without first stripping DRM) and choose the best price. I find it's rarely the Sony store anyway and I agree, their interface sucks.
Eric