Here's an official site:
http://europeforvisitors.com/europe/countries/uk/tower-of-london-ceremony-of-the-keys.htmBut this one has more information for tourists:
http://www.hrp.org.uk/TowerOfLondon/WhatsOn/ceremonyofthekeys.aspxThey both say to WRITE for tickets, which is pretty antiquated, but then so is London! I think that International Reply Coupons are available at any post office.
For other ideas, this claims to be the "Number One Internet Site for London":
http://www.londontown.com/London/London_Ceremonies_and_TraditionsBy the way, evidently you can book online for the London Eye:
http://www.londoneye.com/