UPDATE (Feb 2017): Made in collaboration with members here at The Fleet Workshop, this item is now available as a licensed replica, produced by Roddenberry Entertainment.
@EWilliams has an excellent thread over at the TPZ right now on the 3D model he made of the Kataan probe necklace, which he got 3D printed and cast in brass. I wanted to post here to supply some reference images I've collected and a bit of research.
Source: Star Trek Auctions blog
It sold for a mere $2,706 on eBay via "It's a Wrap."
A jewler has been selling replicas on Etsy for a while:
There have aso always been some not-so-particularly-accurate ones on Shapeways for some time
Morgan Gendel also sells a version at conventions but has been sold out every time I have asked him. I signed up on his list at the convention in Boston over a year ago but he never sent any info. Since I haven't seen it in person, I can't speak to its accuracy, but there is much to be said for one coming from the man himself. If anybody has info on how to buy them from him directly (they may be officially licensed?), I'll be very happy to post that here in this thread too.
Ethan's model is definitely the best and most accurate I've seen. In particular, the accurate radial striations on the dome make all the difference.
Shapeways offers both polished and unpolished brass. The former removes the printing lines that are a well-known artifact of the current state of 3D printing technology. Unlike how the plastics polishing works at Shapeways (which I recently used on my Tuvok meditation lamp collar), the 3D printed metals are polished by hand.
The necklace chain
Everyone is still trying to work out where to source an accurate necklace chain. It could either be brass or tarnished silver; it's hard to tell. I tried to get some varied screen caps of it below in various lighting environments. The cinematography for "The Inner Light" is odd (the film grain is unusually high on the planet shots for some reason, and they obviously intentionally warmed up the color temperature on everything to underscore the dying-sun concept.)
@zredthunder think it's silver. I personally think it's brass, if only because these types of necklaces (the kind of chain used on rabbit's foot keychains) are generally pretty cheap, lightweight, and not used on good jewelry. I would be surprised to see it in a precious metal, even silver. But that's entirely speculation on my part.
All of these are from "The Inner Light"
Interesting to note here at that there is a little connecting loop between the necklace and the pendant itself.