Where do I find free ebooks for my Nook?

I just got a Nook to take on my upcoming trip to thailand so I don’t have to carry books. I got the Nook over the Kindle because you can check out books from your library for free! I tried, and the books have disappeared within a couple days, and I don’t know why. They still show up in my Adobe reader “nook”, but they aren’t in my Nook.
Anyhow, where can I find free ebooks to download, and how do I get them to my Nook? (I’m on a Mac)

Thanks heaps!
The sherpa