In reply to Nic:
This maybe?
http://office.microsoft.com/en-us/buy/product-key-card-FX101850744.aspx
I could be wrong though, MS licencing is heinously complicated.
If so, then the licence card version will not work on a your mac, because the software wasn't preloaded. In fact I didn't think there was a licence card version for mac at all. Remember that office and office for mac are different, don't buy office by mistake (unless you have boot camp).
Also, Microsoft often release a mac version of office one year after the windows version (Office 2007 -> office for mac 2008, 2010->2011) and there is a new version of office for windows out fairly recently. So there may be an office for mac 2014 coming soon. You could wait and see before buying, especially as support for office for mac 2011 ends in 18 months.
You could also consider LibreOffice or Kingsoft. They have most of the features of office, but compatibility isn't always the greatest. But then again, compatibility between office and office for mac isn't always perfect.