Is it possible to get the tyre shop due to come out if you paid $$$ to do so? Would be your best option, then you can buy the key, and not have to rebuy a new set. Plus if you try hammering it out and slip, there will be a nice scratch on your rim. You can always try drilling out the thread, just remember, $50 titanium drillbit is required otherwise the normal ones go blunt (I've done this before). Then you've gotta get it rethreaded. Having said that, if you've got to do it to four wheels, forget it, get the tyre man to come over with his lock nut keys.