Go to Primm, to Nashes' House, and save the game.

Play Caravan with Johnson Nash. If you lose, load and play again.

If you win, you will score caps. Then, open the Barter screen with Johnson Nash and buy the items you need.

Then,save the game and play Caravan again. If you win, you will take back the caps you just paid, but your items don't leave your inventory!

Repeat this as many times as desired.

Note: If Johnson Nash runs out of caps, wait a few days and he will have the caps.