John Avello at Wynn probably has the best reputation for maintaining a Kentucky Derby future book. Lucky's does a very good job as well. Just beware of joints that only lower the odds on horses. If you don't see any horses above their opening line, you are getting ripped off. A good bookmaker will raise odds on horses getting no action to entice bettors.
Lucky's Racebook 2010 Kentucky Derby Futurebook updated January 12.