Where do i buy them? I have only found one in the general store in good springs (varmint rifle silencer) and one at silver rush in Freeside (laser rifle focus).

Who has the best/most gun mods?

Check with Gun runners near the Strip, i think mods are rare and sometimes expensive, i havent found any in containers or anything so i assume they are a rarity. Travelling caravans might have some aswell i suppose :shrug:

Blake at Crimson Caravan, general store in Goodsprings, Silver Rush in Freeside and Gun Runners like mentioned above, I hit these guys up for mods and most of my trading.

The gun runners just south of freeside have the best probability as well as the former NCR soldier on the overpass (its called the 188 or 18something). However, the weapon modifications they carry runs on a random selection that may or may not produce the weapon mods you want. If you really need a particular weapon mod, sleep a week (they restock on Sunday at 0:00) and then go through them all again.
Also the weapon mods are tiered (ie tier 1 has varmint rifle, 9 mm modifications where as tier 4 has sniper rifle mods and the like). If you are looking for the extended clip for the varmint rifle then keep checking good springs or the NCR southern outpost (the bartender in the back) for those mods.
Hope that helps.

ps: if you're on the pc and don't want to go through that much probability (I didn't like it) you could modify how many times the random selection process runs. Chet at good springs runs the random script twice at 0:00 on sunday, I modded it to run 8 times instead so that I was 4 times more likely to get different weapon mods from him. Makes it much more likely that I can get the 9mm scope and 9mm extended clip in the first week when the game starts. First play through without it, I went 10 weeks in game without ever seeing either of them in his inventory.

Thanks guys ... i was unaware that the vendors restocked ... i will check back @ goodsprings and silver rush as well as investigate the new vendors you mentioned.

All weapon mods (in GECK) are part of the vendor's "stock list" and the game randomly picks from said list when it resets the cell (and the actors / objects within that cell).

Some weapon mods are part of "tier#" lists - which I haven't puzzled out yet. That may mean that some things will not show up until you either level up or progress through the storyline.