There is not a DVDrip of 21 available yet, if you look you may be able to find a CAM version, but I never bother with those. All-in-all isohunt is one of the best places to search for torrents
In the mean time, "The Last Casino" is based on the same true story, but is not a hollywood film
The Last Casino - MIT Students - 21 - Blackjack - DVDrip Xvid | 21 | isoHunt - the BitTorrent and P2P search engine
Edit: I suppose I cant mention good sites to find torrents without bringing up Superfundo -
SuperFundo | The best source for movie torrents. Always good quality and new movies are usually available within days of being released on DVD (Sometimes they get pre-realeses as well, such as Jumper)