this might help some people, feel free to add any i missed guys.
:fallout:
contreras at camp mcarren - huge supply of .308 ammo
gun runners - most ammo and weapons good supply
mick and ralphs in freeside - speech check for upgraded stock
briscoe in novac - decent assortment usually
mojave outpost - minor supplier
great khans armory - major supplies of ammo
quartermaster seargent at hoover damn - major supplier
silver rish casino - ok for microfussion cells and getting weapons
trading post 188 - has several merchants with ammo
nash in primm - minor supplier
chet in goodsprings - minor supplier
nellis air base - the boomer in the hanger has some usefull stuff
the guy at the ceasers legion base - if you're not enemies with em
:ph34r:

Van graffs

The guy who sells the concealable weapons to the right as you enter the Strip - I can't think of his name.

Mr Holdout

Brotherhood of Steel quartermaster : good EW ammo, very limited gun ammo