John, beware of buying sextants on eBay and amazon. Many are replicas made in India intended for decoration in personal libraries that have now entered the market as the real deal. The most reliable dealer for sextants is Celestaire.
The Astra IIIB and the Tamaya are top notch. We have one of each. Bought one from Celestaire the other via eBay. We 've used them to take noon sites but now they are pretty much gathering dust. Nevertheless, we still take them with us when crossing oceans just in case (having been struck by lightning on a previous boat we know what can happen to electronics). We also have Mary Blewitt 's book, a pocket computer that has a perpetual almanac in it, and Star Finder 2102-D, which are very useful accessories. Good luck.
Why not post a note in the eBulletin and in the for sale section of the forum to see if any member would want to part with theirs? Where are you located?
Vice Commodore, OCC