You have to sideload them to the Nook into My Documents. There is information in the User Guide regarding sideloading and I believer there also might be a video on it.
Unfortunately, we can't post links to other sellers on these boards, as they are afterall, the B&N Boards. However, you can find a number of them by googling ebooks. Just remember that you can't purchase from Amazon and you need ePub or pdf.
You will also need to download and install Adobe Digital Editions (ADE) to get some of the books. Look in my signature, there is a link to a post from FrogAlum about installing ADE.
If you google, you might find some "too good to be true" prices. There are people out there trying to sell "pirated" versions.
Also, use the search engine on this board to look at older posts. There will be mention of other sources in these.
That sideloading video is at nook.com - QA and Support. It's worth watching.
Thank you very much.
I copied Doug's and Frog's pointers and instructions onto a Word doc so I could find it again without going through all these board messages!
There are also other boards and nook communities than these (though these are definitely a good strong community) try a google search and you can find some other nice places where you can meet more nookites and ebook fans/writers like the ones at nookboards and other sites.