Spanish proofing tools for Office XP

A friend of mine uses a Dutch version of Office XP (legally bought). Since he
has multiple friends in South America he wants to have the Spanish proofing
tools installed also. However, the Spanish part of spelling/grammar is not
part of the standard Office XP NL-version CD.
Where can these files be bought, downloaded ? Microsoft only states that
the proofing tools cannot be bought on their website, and doesn't give any
hint where to buy it. Not the most customer friendly attitude.