Henry Mero said
Where would be a good place to go looking for a 1866 rifle sight  

Good luck Henry.  They look almost exactly like a Henry sight.  The last version of the Henry site is what the 66’s have.  Most people don’t know one from the other.  It is not that uncommon to see a single v notch sight on a Henry or 66. I’m not sure who made them but they are period correct.