Look at a few videos from some of the worst parts of Africa. Yeah....lots of AKs.....not so many shoes.
AK47s can be bought for a chicken or two in some parts of the world. They can be hand built. Someone even built one from a $2 shovel (I think he purchased some other parts, but he could have done it himself.)
So, yeah, very easy to get an AK47.
Most of Far Cry 3's story explains that various groups brought over weapons and usually left them there.
Far Cry 2's weapon degrading would have been a good addition to this game.