Tickets can still be had for the 2011 Final Four in Houston, you just need to know where to look. Here are some places to get individual game tickets, all-session passes, and complete travel packages.
VIP/Hospitality Final Four Packages (including hotel)
Sell tickets through the NCAA's official exchange
No long the "official" provider of hospitality packages, but still a decent option.
Individual tickets & packages. Some decent low-end seats/prices.
Hard to get tickets, though pricey.
Ebay.com - search for "Final Four"