Are there any weapon mods scattered around the wasteland? I haven't been able to find any yet. And all the ones for sale are like 500 caps.

Figured this out last night.
Most vendors will have weapon mods.
If they have 1 or 2 but not ones you want rest for 48 hours then check their inventory again..... seems the mods change every 48ish hours
Stood at chet in goodsprings and rested until I got my varmint rifle all decked out.