Chet in Goodsprings has the highest percentage chance of spawing it for you. He has a chance to spawn 2 of the following: 9mm scope, 9mm mag, Varmint Rifle scope, mag & silencer, .357 cylinder and .357 barrel. So you've got a 2 in 7 chance for him to spawn it each time he resets his inventory. Here are some instructions you can follow:
1. Wait (by using the in game wait function) for 3 game days.
2. Travel to Goodsprings.
3. Save outside of Chet's door (not once inside, you'll want to do it before his inventory spawns)
4. See what good 'ol Chet has to offer.
5. If he doesn't have it, reload the save and try it again.
If his inventory doesn't seem to respawn, you'll want to do this:
1. Enter his store.
2. Make a hard save.
3. Completely exit the game.
4. Start up game and load the hard save. His inventory should respawn then.
5. Keep reloading the hard save until you get the mod.
Hope this helps.
-Gunny out.