Laser sights have come a long way since the bulky, power-hungry and expensive models the Army used 30 years ago. They've slimmed down, become self-contained, and are much more simple in their design. In fact, they're so simple that you can build one for your revolver pistol in a matter of minutes from a toy laser pointer and a generic mounting bracket.
Most toy shops, hardware stores, hobby shops and speciality stores sell laser pointers. Since you're using it for a short-distance pistol rig, you probably don't need one of the higher-wattage lasers that can be seen by low-flying aircraft. You'll want to get one that's about as long as an average index finger, or about three-quarters the length of the pistol barrel. Make sure the laser you choose can be seen clearly, not in its beam but in the termination point, from at least 100 feet away, easily testable in most stores.
The Mounting Bracket
Pick up a generic laser mounting bracket at any sporting goods store that sells firearms and hunting gear. Make sure it is the type meant to hold tube-shaped lasers about the length of the laser pointer you picked out. It should have at least two adjustment screws built in that will allow you to trim the laser's path to accurately predict where your bullets will land. Make sure it's also rated to fit the firearm you're trying to upgrade. Revolvers can be tricky to fit with laser sights, so ask an expert in the store if the bracket you've chosen will work.
Mounting the Laser
The bracket will enable you to secure the laser against the barrel of the pistol, either underneath or on top. A series of screws will need to be tightened to solidify the laser's position, and at least two others will be able to adjust the accuracy. Make sure the laser is firmly attached to the gun, as the recoil of the weapon is notorious for dislodging shoddily-installed components.
Activating the Laser
Homemade laser sights can be activated in several ways. Consumer laser pointers always have an on/off switch on them, but usually it's in an inconvenient place for activation while holding the revolver and trying to aim. If the laser pointer has its activation switch at the back of the unit, mounting it beneath the barrel and near the finger guard might offer you possibility of activating it with a free finger. Others more innovative have installed an on/off switch of their own by tinkering with the laser pointer's circuitry, or held the switch down with a bit of tape or elastic prior to target-shooting.
Adjusting the Laser's Accuracy
Every laser mounting bracket has screws that allow you to adjust the direction the laser points while mounted. If your bullets aren't hitting where the laser says they should be, adjust the laser's direction as necessary and try again. With a homemade laser sight, since the laser pointer wasn't made to fit inside the bracket, the adjustment screws may be useless. If they are, just use epoxy to solidify the laser in place, adjusted correctly. This means the laser can never again be readjusted for a different gun.