Sony is switching their entire ebook store to ePub tommorrow, see Epub coming to the Sony Store on 11 December
. All their ebooks will be readable on the EZ Readers, and also on the B&N Nook. This may be the cheapest single source for US-rights ePub ebooks, although recently they have typically been more expensive than Amazon and B&N (which don't sell Adobe ePub).
Ebooks with DRM from the B&N store are not directly readable on the EZ Readers. B&N does seem to be trying to compete on price with the Kindle Store, and it is possible today to buy B&N ebooks in DRMed eReader format (e.g. for a Windows PC or a Mac) and then strip the DRM so that they can be read on an EZ Reader. In the US, this is probably legal if you bought the ebook and create the DRM-free version for personal use.