You guys are correct. Both the Z68 or the crenelated defender tailcap will work just fine. As you said, the "brains" of the 2-stage output are in the head not the cap.
In the past, I got my Z68s from either Surefire direct or Brightguy.com. In another post
(here) someone said that SF is out of stock, and as of this morning, Brightguy website is saying the same thing. I would suggest doing a google search for Z68 and just go down the list.
cl0123 has two good alternatives. As far as that picture that is shown on the Optics Planet website, I wouln't pay any attention to it myself. That's just an inaccurate website depiction of the Z68. There is only one Z68 and it is NOT crenelated. That's just an old picture being used. In fact, on the back of the package of a brand new Z68 that I have, you will see a small diagram showing how to replace your tailcap with the enclosed Z68, and the diagram actually depicts a crenelated tailcap even though I am clearly holding the smooth, round, non-crenelated Z68 in my hand.