PanMacmillan sells its own ebooks on-line in ePub.
If you want to stick to Adobe ePub, Books on Board is your best US source. If you don't mind stripping the DRM (from LIT or MOBI or eReader), then there are many choices of US stores (BoB and FictionWise are the most often recommended here) and a much wider selection. FictionWise also has DRM-free ebooks in Sony format.