I have to say that audiobooks are probably that hardest to come by and my favorite type. My main source for audio books is Librivox. They only carry public domain books and classics...but there selection is huge!
My favorite source however is the Books Should Be Free website. They offer like 3000+ titles in audio plus various ebook formats! :)
Well, if you filter the Amazon kindle section from low price to high, you'll get a ton load! You should check often though because some authors put their books for free for like only a week. This will probably work with any ebook store with a price filter. Another good place to look is Smashwords! So many there!
If you look on Booklending.com, you can loan and request to borrow kindle books.
Project Gutenberg, haha the mecca of free books!
Also, check out these freebie newsletters: Bookbub, FreeBooksy, Book Gorilla & Pixel of Ink.
For pre-released and review copies, you can check Netgalley and Edelweiss.
You can of course always go to a used book store and trade... but its not usually a one-to-one trade.
You and geo search for free books using Bookcrossings.
You can trade online using Bookmooch or Paperback Swap.
You can search through the giveaways on Goodreads or open a bloglovin account and follow all your fave blogs to keep up with their giveaways.