Mod Master

 aka Mod Master is an achievement/trophy in Fallout: New Vegas which is awarded for successfully attaching 20 mods to weapons.

Gun Runners, just East of New Vegas usually sells several weapon mods and they get new mods every few days. That would be a good place to start.

The merchant inside the Crimson Caravan Camp also has several weapon mods for sale. Usually 3-5 different ones.