By Buying the desired game from www.ebay.com. or maybe on steam.com
No wrong. My son has a Nintendo DS that has WI-FI capabilities. You need to get a Nintendo DS, and a compatible game with WI-FI, you can go to the website in the web to look for people giving pokemon away or trading, or you can do it from within the game. I know my son does it all the time with Diamond, Pearl and Platinum.
LINK: This is the website you need to check out: http://www.pokemon-gts.net/