I had a NECG Ruger peep, it fits on the scope mount cuts, and it is a nice piece of work. IIRC I got it directly, but Brownells has them too.
It wasnt on a Ruger for me (a custom application) and went with the gun when I sold it, so I cant attest to how it would work with the std. frt. sight. My method of checking that is to mount a peep and adjust it to look through the open sight set, then remove (or if possible, fold) the open rear.