Check out http://www.nashbar.com/. They have a large selection of wired and wireless computers. Wireless gives your bike a cleaner look but can be a hassle because you have to have transmitter and receiver aligned correctly for it to work. You might also run into radio frequency interference(which isn't always a bad thing....."Wow my average speed today was 130 MPH!").
Wired doesn't look as good but you don't have to worry about the wireless issues. I personally use Shimano Flight Deck, partly because my bike came with the compatible Flight Deck Shifters. This system gets a little complicated because you have to enter the number of teeth on each gear for the cadence to work. It does not measure real cadence but figures out what the cadence would be for a given speed and the selected gearing, sort of a virtual cadence. I get some really radical cadence readings when I am coasting down a hill with a low gear selected and have'nt spun the pedals at all.