Any bike shop, Performance or REI (they have online stores too!), or other online bike supply places (like Nashbar, etc) should have them.

Cateye makes a variety of wireless models with a range of prices, and some of them are pretty highly rated. I have a Serfas one, which I don't particularly care for because it's far too easy to reset and prone to interference. Well, all wireless computers are prone to magnetic interference. Just the nature of the beast.