WHAT MODS ARE:
Weapon mods can be found at various vendors throughout the Wasteland, but the best place to go is "Gun Runners" near the New Vegas Strip. They usually have a lot of gun mods in their inventory under "Misc". You will need to have the gun you buy the mod for in order to install it. Note: Every vendors' inventory recycles over time, so there are more than enough opportunities to collect weapon mods. If, however, you want to get this done as fast as possible, there is a way to "force" a vendor to change to new mods. See the bottom of this solution for directions.
Here is the location of Gun Runners (JPEG): HOW TO INSTALL:
To install a mod, go to the Weapons section of your Pip Boy and scroll to the weapon you bought the mod for. Click the LB (Left Bumper) and a list will open up of available mods for that weapon. Just click A on the mod you want and the next time you take out your gun you will notice your shiny, new addition. These mods do not
need to each be unique for the achievement. You can use duplicates of the same mods or on the same weapon.HOW TO FORCE NEW MODS:
Vendors' inventory changes every 72 hours or so, so press the Back button and wait for ~3 days and then check with the vendor again for new mods. You can also try the following:
(This was posted as a comment by "cradle crue")
if u buy somethin then save then dashboard then load game back up u have new items(mods) but it does not always work cause few time i loaded up my last been same items.