mazon, ebay and ms online store are the best sites to get the key with no concerns. 100% genuine.
but if you don't have much budget for this, it will be a heavy burden, right?
i purchased a windows 7 ultimate product key last year from EDITED, and it still works till now with all updates. meanwhile, the price is much acceptable.
when i visited the site at the first time, i quite doubted since the price was just $30. but i decided to try as they used paypal(i like paypal very much. paypal always help me protect my right).
the fact proved my decision was right completely. the key works till now. so i recommend it to you. if you have paypal account, it will be perfect.
if no paypal account, you need to register an account with your credit card.