You can buy them from a company called Celestaire in the US.
AFAIK their website is:-
Celestaire, Inc. - Navigation Equipment
A few comments, if I may?
For low-level work (seaplanes & airships, blimps etc) a marine sextant can be used if there is a visible horizon (with appropriate corrections from the Nautical Almanc for Dip, refraction etc), or a marine sextant artificial horizon attachment (such as the Weems & Plath unit or the hard-to-find C. Plath units) can be used with a marine sextant, although with reduced accuracy due to the motion of the aircraft making it very difficult to centre the bubble and celestial body at the same time. The sextant errors (error-of-perpendicularity and side-error, as well as any index-error) would have to be deteremined and reduced as much as possible, although some marine navigators feel a little bit of side-error isn't necessarily a bad thing when doing stars.
The two preferred marine sextants would be the Tamaya Jupiter (or more expensive Spica) models, or any of the Plath, C. Plath or Weems & Plath full-size marine sextants. These can be bought new, or from reputable second-hand sextant dealers (such as Joel Jacobs in the US; his website is
Land and Sea Collection Nautical Antiques,). Tamaya Jupiters are standard on the bridge of thousands of merchant ships and are a fine sextant at a much more realistic price than the no-longer-made C. Plath units. (I have owned and used both.) Cassens & Plath sextants are still available new but are likely to be significantly more expensive than the Tamaya for no discernable increase in practically-achievable accuracy.
However, for work in a high-flying aircraft the purpose-designed and built aircraft bubble-sextants, taking 60 readings in a minute and averaging them, would be a far better choice. You will also need to get the "Sight Reduction Tables For Air Navigation", and "Selected Stars Epoch 2010", unless you are working your sights using a calculator.