A couple of options, next time you are out and about running errands and such stop by the LGS especially those that offer scope mounting and ask if they have the plug screws...
Most online gun part dealers carry them, try Brownells, midway USA etc. As mentioned above you can call Ruger service department or order them from their store.
You can also mount an inexpensive scope base (weaver) for less than 10 bucks that will plug the holes and not interfere with the iron sights and be there should you decide to mount a scope.
They plugs aren't necessary for the rifle to function, yet should be there if for no other reason than to keep the threads free of gunk and grit.