check this out guys...cheap ac/dc adapter! voltage selector ~ 3v, 4.5v, 6v,7.5v 9v, and 12v
Jwin Jac32 Egg Power? Universal Ac/dc Adapter (current: 300 Mah, Power: 6w) - GeekBro.com
havnt purchased one yet but this seems to be the ticket if you want to have variable fan speeds.