Best boat compasses according to redditors

The Davis 15 and Davis 25 are common introductory sextants and about the cheapest bet for a real sextant. A good metal one will run you over a thousand although you can find vintage WWII surplus (every bomber carried them avoid aviation sextants apparently) ones for markedly less but make sure it isn't a reproduction.

Alternatively, if you are handy look at the book latitude hooks and azimuth rings for a how to on making and using a ton of historical navigational tools including the sextant's very close cousin, the octant.

It will be low enough quality that you can't get accuracy better than a dozen+ miles. Enough to learn from. The Davis MKIII @ amazon linked above is actually used on board ships as a backup and is as cheap an instrument as is usable for navigation. A "commercial" grade sextant will be over $150 and up to $2000. As usual, you get what you pay for. Its function and accuracy depends on both mechanical and optical accuracy.