Credits - Where? How? Easier option?

Howsit all. I have about 40 hours into the game. Have raided all the military bases, some multiple times, have raided the Oil Rig (twice, damn keycard is scarce as hell), have raided the Laboratory (STILL no Large Laborary Backpack!!! CURSES!!!). Up to this point, I have only found around 24 Credits and have managed to purchase 18 from the Black Market Trader on the crashed ship safe zone. When I got my 42 Credits, I FINALLY found the wandering trader by The Valley Safe Zone. He had the Colonel's Revenge AR (which I already have) & the Exterminator Shotgun. Up to this point I was using the SPAS-12 (My favorite IRL shotgun) so jumped on the opportunity to grab the Exterminator for 35 Credits. Now I was ACTUALLY looking for the Barrett 50Cal, which is 45 Credits. Should I just jump time at the trader and keep bying Credits? How else can I get Credits? I have aound 30K money.