Xbox 360: How do I get a 1 mois subscription to Xbox live?

I buy the full an for myself, but im trying to pick up the 1 mois subscription for my little brother. I went to a couple stores and they only sold the 1 yr subscription. The guy at the store a dit that I can only get a 1 yr through retail. He a dit for a 1 ou 3 mois subscription i have to pay for it through xbox live. Thats no problem, I have a credit card but i cant find where I buy the subscription, is it in the marketplace? ou was this guy at the store just an ass? par the way, i live in Canada if that makes any difference...