The Arduino USB and serial boards needs a power adapter with approximately these specs:
- a 9VDC to 12V DC 'wall wart' adapter, rated 500Ma or more
- the connector is a 2.1mm dc power plug,
- the connector need to be 'centre pin positive'
A possible part number in North America:
JAMECO Electronics (California)
Jameco 202201PS $4.90 USD in quantities of ten or more.
That said, you probably have something lying around that will work. Find a plug that fits the jack, with bare wires. Then, find a 9 or 12V DC 'wall wart' supply, and connect its wires to the connector's wires. The centre pin must connect to the positive side of the power supply. Reverse polarity is not a huge issue, since the Arduino incorporates a blocking diode to protect against this-- but you might want to remove the ATMEGA chip while gettting things right.
Of course, if I have any of this wrong, Massimo will correct me.. Massimo?