A lot depends on what you're looking for. There's Dune, the first book. Then there's the first six books written by Frank Herbert and then there's a bunch of sequels co-written by his son Brian and Kevin J. Anderson.
I'm more of a "purist" so I go by the original author which means you'd want the 6-book saga.
Penguin is going to publish them and /u/DrNSQTR provided a link but otherwise you're going to buy them on Amazon or Ebay. I found this on Amazon which is the first 6 books in paperback for $65. It's a reasonable price at about $10/book.
$73 on Amazon but that's for six books so pretty reasonable.