I had the same problem with my first playthrough.

If you want top-notch weapons go to vault 34, you can get the unique marksman carbine, a
riot shotgun, some assault carbines, a pulse gun just to name a few that place is a f'n
goldmine. All you have to kill are some ghouls. Of course though, where there are ghouls
there is radiation, so stock up on rad away. Check out youtube for a walkthrough to get to
the armory, and the location of the vault. If you can't find it lemme know i'll do my best
to walk you thru it.

Also check out the silver rush in freeside, there's lotsa energy weapons/ammo on display.
Use the grab function to drag the weapons to a safe corner where you can remain hidden and
steal them. They are in poor condition though. Also you can buy a Gauss rifle at the siver
rush.

You can also visit the gun runners for your weapons needs but it's very expensive.

When shopping at a vendor and you see that he/she has 2 of the same weapon buy the
cheapest one and sell it right back do this a few times and you'll notice that the weapon
is now at 100% condition, this can be used to buy everything the vendor has that you need
or want, it can also be an good way to fully repair some of your weapons.

There are many other great guns but most of them are in tough place like a light machine
gun at black mountain or lotsa gauss rifles in the BoS bunker, if you can survive there to
get those weapons and leave alive, then you are no longer in dire need of upgraded
weaponry.

dont travel long distances go by towns try 2 get 2 1 then 2 the other untill u get 2 the
strip then but stock up on whatever u find the cowboy repeater is a rlly good gun 2 start
with also dont underestimate the power of a fully repaired 9mml pistol