Also, a fair amount of the western weapons probably come from all the converts and then get redistributed to important squads closer to the center.
What I'm saying is that even in the SoC and Clear Sky maps there's a shittonne of territory between the maps that is grayed out. Going there is out of the question because of gameplay reasons but in the lore there are paths that hard to find but which can be navigated ?
For example : How did Garik find his way to Pripyat in CoP ? He sure as hell didn't take the tunnel route so he found another way. The Monolith must be the same and has numerous scouts and guides find paths through which to smuggle in goods. Then there are the Death Trucks which are used to bring Monolith agents to stalker hubs so they can be used to transport food and other supplies as well.