Sites like slickdeals and others will show if there are any good offers.
Also this isn't a bad way to find one either.
http://www.google.com/search?client=opera&rls=en&q=xbox+live+12+month&sourceid=opera&ie=utf-8&oe=utf-8&channel=suggest#client=opera&rls=en&q=xbox+live+12+month&oe=utf-8&channel=suggest&um=1&ie=UTF-8&hl=en&tbo=u&tbm=shop&source=og&sa=N&tab=wf&ei=CjYoUKSuNILJqgHK2YHwCQ&bav=on.2,or.r_gc.r_pw.&fp=1d355f436f4a91c5&biw=1280&bih=687