I have a Nook, and I buy books from a variety of different sites. Most places offer epub format these days, which is what the Nook reads. I buy some books from the Nook's built-in B&N store, and others from
Kobo,
Books on Board, and
Fictionwise.
I don't think it's possible to self-publish via the B&N store like you can with with the Kindle store. (There are, however, sites like
Smashwords that let you do the same thing but aren't connected to any one device.) I don't know about the iPad.