PLEASE DELETE - I found information on the forums that the ammo for the SKS changed. I do share it here for you:
"Mosin dont take 7.62x51 anymore, they now take 7.62x54R, and the 7.62x51 has been renamed .308
You are looking for 7.62x54R ammunition for the Mosin
.308 Winchester is for the Blaze and Longhorn (not to be confused with .380 which is used in the Makarov and that little SMG that nobody can ever find, or .357 which is used in the Magnum and the Repeater.
I think residential spawns are still best for finding Mosin ammo, since it is out of service everywhere in the world as a military service weapon and is now commonly found in private hands.
You might find it as well in the areas that we generally used to think of as "military" tent camps. These areas are now spawning civilian and residential type loot...
...which makes a lot of sense if you stop to think about it. These areas never made much sense before when we all assumed they were military camps, but are now perfectly logical if they are actually emergency evacuation camps or refugee centers meant to help the civilian population as the infection spread and the crisis deepened."