Here's a list of npcs that you can challenge. The ones that cost less MP are generally going to be easier (depending on the rules they have set).

http://ffxivtriad.com/npcs

I'd recommend starting out getting the cards from the lower level npcs, and moving up when you have better cards to use.

Also, if you have the MP to spare, you can buy packs from the vendor npc, next to the Triple Triad Master.