Upgrades can be found in specific locations or the same locations throughout the game. Although later in the game they can also be found off infected and seem to be more random in nature.
Thus this gives the explorer type player, a chance to level up all weapons and their upgrades to MAX.
sug: upgrade most weapons as soon as you can - gives more room for ammo. Exceptions: standard pistol, do not upgrade capacity. when you die you, the new character has this weapon and the standard bullets even if upgraded. you loose any extras in it.Empty it first & if you die you get 6 free bullets. all others except for 2, don't upgrad the firepower of the crossbow. the crossbow is already a 1 hit 1 kill weapon if you hit the head. you can recover the arrow. if you hit but miss the head then switch to another weapon, it takes longer to reload than to switch & if you hit with a 2nd arrow you still only recover 1 so you loose the other. Don't upgrade to assault rifles RoF, them & the SMGs are ammo hogs anyway & a short burst without the upgrade will rip any Zombie apart anyway (or the head off a riot Zombie). upgrading the RoF of these weapons just burns threw your ammo faster. I upgrade the SMGs & take them on the final palace mission for the bosses there.