if you still have an original xbox, you're pretty much out of luck. you have to call xbox live. i tried going to like 3 best buys and a couple circuit city's before to get a subscription card for regular xbox but they don't sell them anymore. or you have to get your hands on a two month. i gave my last one to ngsm so i don't have anymore.
do you have the platinum ($400 expensive one) 360? If so then it comes with a one month of gold.
One thing i always thought of but never did:
go to best buy, open a 12 month box and write down the code. Walk out, then use it at home. I mean, bestbuy employees are lazy so they wont come to you, and it just looks like your looking at stuff...maybe i should try this...