I was afraid that would be the case (easy to obtain). Quick search proved what you said to be correct... they are available on Amazon (not going to provide link as I don't want my posting it to be used by someone for criminal purposes). This is fricking ridiculous!!!! Why the heck sell the locknuts if the master key sets can be so easily purchased.I used to work in a dealership and its insanely easy to get a masters key.
Haven't checked for myself but I'm sure a 5 minute search on Amazon or eBay would turn them up.
At the end of the day these won't stop but slow down thieves..