Its called a  "Key" and can be found at most hardware stores, all plumbing supply stores, and Home Depot and the like.  Before buying one test for the depth of valve.  Get a key with a long enough handle to reach the valve.     

Remember when you use it the valve will open and shut with only a quarter or half turn.  Don't force it.  Break it and you have a big mess with a lot of digging this time of the year of very wet cold dirt.