I've got the 12" and use them all the time. This is a great price for the longer clamps:
http://www.amazon.com/24-Irwin-Quick-Grip-SL300-Clamp/dp/B001M0W1YA
or check the Rockler site
http://www.amazon.com/24-Irwin-Quick-Grip-SL300-Clamp/dp/B001M0W1YA
or check the Rockler site