Welcome bbean. There are many free books available for your Sony reader right here at MobileRead. And, you can get a hundred or so Science Fiction and Fantasy titles at the
Baen Free Library. Baen also sells additional titles through its
Webscription site.
Fictionwise sells a portion of their titles as non-encrypted (multi-format) books ready to be read on your Sony reader. And, there's always the Sony store. I believe that there are other online stores in the UK selling books for the Sony reader, but I haven't investigated them yet.
Welcome to MobileRead and I hope you really enjoy your reader.