It does depend upon which format you are trying to buy the books?
I and other UK membors have had good luck buying from
www.fictionwise.com and
www.booksonboard.com, and they've never restricted us in which books we can buy.
Most either purchase .lit or mobi files and reformat them for the sony.
I do believe that the Adobe books can be loaded onto the Sony using Digital editions, but someone who has done it will have to confirm.