The good news is that even before you’re allowed into Ammunation, one is waiting to be discovered in Los Santos in an easily accessible location which doesn’t involve any climbing of fences, wanted stars or armed guards to reach it.
If you head north to the area indicated on the map – it’s the road that runs to the north of the metropolis in Los Santos before you head up into the hilltop houses. Turn up a ramp and then sharp left and in a courtyard tucked away in the corner is a batch of ammo for the AK-47.
Each time you save the game or start a new mission it will re-spawn and you just need to head over there on your way and you are well on your way to doing some serious damage on the early missions.
There are others available in Los Santos, but this is the easiest to find. For example there’s another one behind one of the Vinewood movie studios.