Den six (at all three difficulties) has the ability to drop BB’s. BUT they were rare, maybe one every 40-80 runs. I believe they recently increased the chance for them but I can’t say for sure. I’m going off what my mate showed me.
When the XP “update” came, I recalled farming from 50-90 and literally seeing 2 BB’s.
Anywho, like the other lad said, you can get them from any normal mission, just a low chance.
Overall, if you want BB’s, farm RB normal, highest BB drop chance imo.