Wheel/tire shops have that type of key lying around because they're common, not all shops but most, just cross your fingers its the same key as yours. In which case you're good, you just have to buy their key or new lug nuts and key.
If it doesn't work that way, well you're in a pickle, but you can get this tool:
http://www.ebay.com/itm/Lug-Nut-Remo...048540&vxp=mtr.
You just have to buy new lug nuts as the tool destroys them.
-RyanG