http://www.mezzobikes.com/UK/accessories.php
their commuter bag is pretty impressive. but you need to have a rack similar to their's, so the bag can clip into the rack.
strangely, the commuter bag is only listed on the UK version of the site. the brompton rack bag is pretty awesome too, but a little slower to get on and off because of having to clip the strap in the back around the rack.