Go to Paradise Falls and meet this guy named Grouse.

He will give you a quest, do it and there is a key in Eulogys Pad on the small table near the bed.

Steal it and open the slave pen, then go to the adult slave pen.

Talk to one of them and then go to Grouse.

He should just talk, but then you talk to him and get more caps.

If you got all of them you should get 1,000 caps each time.