Ok, that's different.

What barrel do you have right now cause it will determine if you can install rifle sights.

If your barrel has a vent rib then Williams makes a set of rifled sights that will mount on the rib very securly. They are called the universal "slugger" fire sight.

Midsayusa has them for $30 (part # 671-251) or any gun shop should have them or could order them for you.