Is it possible to craft any mods at all in FA:NV or are you purely in the hands of RNG in regards to where you find them? So far I've really been down on my luck on mods to Varmint Rifle. Chet does NOT supply them in my game.
I am very sorry but no, weapon mods cannot be crafted, they can be only be bought from merchants (or spawned using the console).
Chet is not a particularly good supplier of weapon mods, and usually only ever carries mods for entry-level weapons such as 9mm pistol, varmint rifle and the laser pistol.
In the standard game, these merchants are your best sources for weapon mods:
Merchants refresh their inventory every 72 hours and their inventory is at least partially affected by your current level.
The purchase of the Gun Runner's Arsenal adds more weapons and weapon mods to many merchants in the Mojave Wasteland.