You have to get lucky and find them. Me and two friends were out an about at Stary Sobor and a clan drove north in a V35 and we slaughtered them. The truck was fully loaded with multiple of each high end weapons including M4A1 SD, AS50, M107, M24, about 5 sets of GPS, NVG, Rangefinders, and the list goes on and on. Plenty of supplies to keep us driving around for weeks fully stocked with ammo and everything. When you get lucky and find one, it's usually been stashed to the limit by people hoarding items inside it. You just have to be lucky enough to find one that's either abandoned, or catching a group of people off guard.