After you have recovered the Abraham Lincoln memorabilia from the history museum and returned it to the slavers at the Lincoln memorial for burning go to paradise falls.
Gain entry by collecting a couple of slaves. Once inside a Slaver there will recognise you and offer you some random items.

Except the items then continue to start up the dialogue and keep accepting the items until you get your fill.

It's stim packs or shotgun shells fifty percent of the time.