IronBuck
NECG makes a peep to fit the Ruger and I think they have a website or otherwise Brownells also stock them.
Even though you don't intend to use a scope, the fact that Rugers have the integral bases and detachable rings means that you could keep a little scope (with rings attached) handy just in case.
Something like a 1-4 or 1 1/2- 5 Leup would be my choice.
Just a thought.