In the US Books on Board
is a good source for Adobe ePub ebooks. In the UK, ePub is the dominant format and I like WHSmith
If you want to use a single ebook store, then the Amazon Kindle is the best choice and the Sony PRS-300 and the Sony store is probably the 2nd best option today.
Once the EZ Reader's get eReader support, they will have a wider selection of stores to choose from (without striping DRM) than any other EInk devices. At that point the B&N store could become your primary source of ebooks. Choosing a device based on future capabilities is dangerous, but in this case the Pocket Pro already has Adobe PDF/ePub so eReader can be thought of as a bonus.