If your car is recent there is a good chance that your main dealer will have a master set of nut keys, if the nuts were fitted by the manufacturer.....failing that, a firm called Irwin does a kit which essentially gets hammered onto the offending nut, it cuts its way into the steel and allows the thing to be undone. Available from motor factors etc.
John
Usually nuts come with a card giving the key number of the nuts. Often that card can be found with the handbook in the glove box or written in the handbook.
I threw my locking nuts away and put the normal ones back on. I'd rather have my wheels nicked that lose the key or be stuck in the rain with a puncture and a key that's slipping round the nut.
As John says, you can get locking nut removal sockets from machine mart or the likes for around £15 a set.
Failing that, you can usually get them off by hammering and old socket over them
or go to your local fast fit centre and ask them to whip them off,iro £10-£20
then give yourself a slap for losing the proper socket
if buying a new set look on ebay unless you can get a set cheap locally?
I`ve always removed locking wheel nuts by smashing a just under socket onto them, if they are the solid type, if they are the ones that spin on the outside a hammer and chisel is the way forward