https://www.adafruit.com/products/775  for $17.50
Shouldn't be to hard to mount a plate to it, There are larger ones as well.
Hope it helps you!

Slip rings.

_____
Rob

this seems to b 1 method to do it:
http://en.wikipedia.org/wiki/Slip_ring

they claim to b better:
http://www.mercotac.com/html/faqs.html#q1  
(but they r not RoHS compliant because they use mercury...  )

Guys, this is perfect.  Exactly what I'm looking for!  Thank you so much!

Hey guys,
           I'm working on a project where I want a sensor (sonar) to be mounted on top of a fully rotational servo (one that continuously rotates CW 360 degrees).   The problem I'm having however is how can I attach the wires from the sensor to the base of the robot without them getting tangled as the sensor rotates 360 degrees? 
           I know for professional radar antennas they run the wires in the center section down so that it won't be rotated, but even that would require some kind of adaptor. Does anyone out there know how this can be done with a servo?
           I'd really like for it to be one sensor that can rotate 360 degrees continuously, but if this is absolutely not possible, I suppose my best options would be:
        - Have one sensor that rotates 360 degrees, then back again.
        - Have two sensors 180 degrees opposed that go 180 degrees then back again.

Thanks for the help guys!