You can't buy from B&N to put books on the Kindle; B&N's books are mostly epub, and the Kindle doesn't read epub. (B&N may have some PDFs, but I think they're all DRM-infected.)
The Kindle reads non-DRM'd .mobi/.prc format and .pdf, so you can buy *non-DRM* ebooks of those formats for the Kindle. You can't buy non-DRM'd books from any of the large, mainstream-publisher ebook stores like B&N, Borders, Kobo, or BooksOnBoard.
You can buy non-DRM'd ebooks from Smashwords.com, Baen.com (they focus on science fiction books), the "multiformat" books from Fictionwise.com, and http://booksforabuck.com/
You can also get free .mobi books from plenty of places:
http://wiki.mobileread.com/wiki/Free_eBooks has lists of dozens of free ebook sites, both downloadable and read-online ebooks.
http://www.teleread.org/free-ebooks/ is Teleread's list of free ebook resources.
Baen Books has a free library of science fiction ebooks at http://www.baen.com/library/
Sci-fi author & copyfight activist Cory Doctorow releases all of his books as free ebooks at http://craphound.com/
http://www.smashwords.com is a self-published ebook store, but it has a lot of free and "pay what you like" books.