I assume poker = texas hold 'em here.
Good poker games on the DS are few and far between. I know of no homebrew titles but that does not mean there are none.
As for commercial games
42 All time classics/Clubhouse games has a limited poker game, AI is not that bad just the rules are not free to play with.
Sega Casino, probably your best bet here.
texas hold 'em Poker, AI is useless and you will go about 2 minutes before headbutting a wall if you do not turn the animations off but it can be playable.
Golden Nugget Casino - mediocre at best.
The other two titles texas hold 'em poker pack and world championship poker - Deluxe Series I have not played at length
The GBA though has quite a few, ultimate card games and ones with poker in the title are a good bet although the lack of stylus contol is noticeable.