@Sunkmail, one manufacturer of replacement derailleur hangers, Wheelsmfg, makes 260 different hangers so you will probably need to remove it and match it to one on their chart if you plan to order it online. Or take the bent hanger to a local bike shop to find a match.
For a temporary fix for a bent derailleur hanger, check out our Simple Rear Derailleur Adjustment article; it includes a description of how to do a quick roadside hanger repair.
When the rear derailleur hanger on my RadRover get bent during shipping, my LBS was able to correct it without replacement. I'm guessing it really depends on how bad the impact if you'll actually need to replace it. Had the rear derailleur guard not been there to take the brunt of the hit, it may have been worse.