Another suggestion is the Calhoun Cycle site:
Dynamo Hubs and Dynamo Lights

Or Harris Cyclery:
Hubs - Generator - Harris Cyclery bicycle shop - West Newton, Massachusetts

Keep in mind that the power output of a dynamo is typically enough to charge a phone, but not enough to charge a larger tablet. If you need to keep more than just your phone charged you might find yourself needing a battery pack anyways.