On the level called "Intercept Nasri the Arms Dealer" there is an area that you can pick 
up unlimited ammo. When you finish the objective "Defeat Nasri's Guards" do not leave that
room. There are three sets of stairs with a door behind two of the and one with a table 
across the top. In front of the table is an ammo pick up. This pick up will res-pawn 
every thirty seconds indefinitely. The type of ammo is random.

During "Investigate Marburg's Villa", in the garden at the end of the level there are 
two ammo piles on the north and south sides that re-spawn if you move back to the doors.