The Williams WGRS is available from H&R for $25 shipped, see Peep and Firesights in the FAQs, it's the standard sight on the 45-70 Buffalo Classic. As Dennis said, you can use it without the aperture, or you can get a ghost ring aperture that replaces the aperture carrier on the sight, that's in the FAQs link too. For hunting, the Twilight aperture is a real nice alternative.
Tim