my first suggestion is to buy it directly from ms official site: http://www.microsoftstore.com/store/msusa/en_US/home
second suggestion is to buy it from bestbuy or amazon:
bestbuy: http://www.bestbuy.com/site/computers-pcs/software-games/abcat0508000.c?id=abcat0508000
amazon: http://www.amazon.com/pc-mac-software-downloads/b/ref=sa_menu_dgs_sft?ie=UTF8&node=1233514011
third suggestion is to buy it from ms partner, like oggus.com or go2keys.com . some forums recommend the sites.
you can compare those sites yourself, and make your decision.
price, guarantee and service are very important.
price:
ms online store is the highest one, but with no concern.
oggus.com is too low to trust, but worthy to try.
guarantee:
no more words on ms store.
oggus.com states a whole-life guarantee, almost the same to ms store.
service:
ms, a little arrogant, my opinion.
the others are much better.
i ever tried oggus.com about 1-2 months ago, since tomshardware highly recommends this site. and the successful activation proved my dicision was right.
it really saved many bucks for me. very glad shopping experience.