Cardboard sextants are a neat idea for an educational purpose, but you could never use one for real, I seriously doubt it can get wet and continue working.

I bought the Davis Mark 3 after seeing it mentioned as part of the navigation package on a sloop preparing for the San Francisco to Hawaii Trans-Pac race. It is accurate to about 2 minutes of arc and is built for actual use. The sun shades are remarkably good, turning the sun into a little orange ball you can sight. Davis Mark 3 users manual

Another place to shop for Marine Navigation supplies is Landfall Navigation. Sextants there run from the Cardboard Sextant Kit to the Cassens & Plath Horizon Ultra Marine Sextant ("accuracy guaranteed to 9 seconds of arc") for only $1795.00 ... Lots of other good stuff too.