Buy Orb1 and Orb2 keys, or if you have no money, run to the underground immediately beneath the Dome. The keys spawn in the locker room and in the keybox on the opposite side of the corridor.
Go to White Pawn, 1st and 3rd floor. BS spawns regularly, along with 120 Rnd. boxes of BT Ammo. It's not uncommon to come out of there with 150 Rnds of decent ammunition. There's 1-4 rifles to be had as well, they come with full mags of BP ammo as well.
There. That's where you get your fucking BS ammo. You might even find AK's and silencers there. Now enjoy the game, and stop complaining about BS not being for sale anymore. You don't need the trader.
Want more, check Black Bishop as well. Just don't come near the fucking IT storage in Black Bishop or I'll end you, Kkthxbye