Tanks of medical grade nitrous are very hard to come buy and probably expensive.
The next best thing are whippits, which are little cartriges filled with N2O used for whipped cream dispensers. These are available in specialty cooking stores as well as over the internet.
http://www.nitrousoxidechargers.com/
http://www.bestwhip.com/

Oh yeah, you're going to need a cracker and some balloons or a whipped cream canister (probably also sold on those websites) to inhale the gas. Do not inhale directly from the cracker as there is a risk of frost bite from the gas escaping.