I think I'd start with the dealer.

Those are called a key or a lug key. Look the car over in all the places where the owner may have stashed the key: Glove box, door pockets, etc. If it was a used car bought from a dealer, the key will not likely be in the trunk because the first thing a dealer's helper does when they take a car in trade, is to clear everything out of the trunk and toss it. If that person did not recognize what that oddly shaped slug was, it got tossed.

Start with the dealer.

If it was bought from a private party, contact him to see if he may have it.

That item is sold in auto parts stores, often in sets of several different patterns. If you are very. very lucky, you might find a matching key at a parts store, but the chance of that is slim.

Contact a tire shop. They might be able to remove it. If all else fails, there is one way to get it off. Weld a short hex rod to the nut and then wrench it off. If a single handle wrench is used, be sure to support the pivot point else the weld may break. The object is to apply pure torque and no bending moments. Be careful not to weld the nut and stud together. After removal, it is recommended to replace the stud since its metallurgical state may have been altered.

I do not think that a nut-buster will work since lug nuts are very tough and there may not be enough space to apply the device.

if they're factory wheel/locks talk to the dealer. don't know how they do things now but back in the old days there where very few different designs. gm's where color coded. there are several versions of lug nut removers online. tire shops should be able get them off.

Thanks for some ideas... as no way to contact the original owner now! Will tell my nephew to contact some tire shops too for any ideas. What a problem now with something made to safeguard ones tires,,,, but the key missing!

Have you had any luck getting the keyed lug nut off?