i compared about ten online stores, like ebay, amazon, microsoft.com, newegg, and some independent sites, like WindowsProfessionalKeys.com .
you will be assured to purchase from ms online store, but from my once experience, their service it not that good. so I decided not to go there.
the price on ebay and amazon seems a little high. they i google it, and found many sites selling it.
it's really hard to recognize which one sells the genuine key. therefore, i sent emails to several sites i'm interested.
WindowsProfessionalKeys.com was the fastest site to reply to my email. and i continued to ask many questions, about the ligelity, payment, guarantee, and refund policy.
i was very satisfied with the reply, ps. they gave me 10% discount as i need two, aha. so I decided to try to buy one first to check.
in 20 mins, i received my key and download links. what happened? it worked perfectly... and then i bought another one with 15% discount they gave me
really a happy online experience, so i recommend you.