You can get 'em at just about any 4x4 shop or online. Just search for any of the following: "detroit locker", "ARB locker", "ox locker", "lockright locker", etc.
Which is the best is certainly up for debate. I think the only agreed upon fact is that a selectable locker up front (i.e. ARB or OX) is beneficial if you can spare the $.
My personal vote is for ARB front and back...can't go wrong there!