You can get from other places, but not from B&N or Borders. That is OK though because Amazon has a HUGE list of what they carry and it is just getting bigger and bigger. It is very rare I do not find a book I am looking for there. I am not positive, but I do not think the Nook or other ereaders can buy books from Amazon.
However there is an easy way around that. There is a free program called Caliber that manages your elibrary. It will convert from format to format as long as there is no dmr protection.