If you have any friends who have purchased Starcraft 2, they will have gotten 2 7-hour trial keys to give to friends. You can ask your friend to have one for free.
You can use their disk to install, or you can download the client online from the website.
Pirating/stealing the game is illegal. And on top of that, ineffective. Starcraft 2 is an always-connected experience. Even to play the singleplayer campaign, you have to login. To login, you have to have a legitimate key which you have purchased.
You can purchase Starcraft 2 at your local video game store (Target, Walmart, etc. included). you can also buy online directly from Blizzard. go to http://starcraft2.com