Where can I get enough cards to play a game of Caravan?
You can buy a bunch of cards at various merchants, which makes getting a full deck fairly difficult. Alternatively, you can help Ringo, and he'll give you a full deck.
Note added from Viper_sb's comment:
If you buy cards at merchants they do not get added to your deck, they stay in your inventory, but you can move them to any container and pick them up again and THEN they are in your deck.
Source
you can also kill Ringo and grab the deck from his corpse.