The only way to get another is to order another from the locking wheel nut manufacture, using the referance number your meant to keep hold of when you get them
If not, halfords sell locking wheel nut removers. They use a sharp left handed thread to screw/cut into the body of the wheel nut. I don't really recommend these, not sure how well they work and will require alot of force.
Or
You can try getting a socket which is slightly smaller than the body of wheel nut and hammering it onto the nut. The idea is to get the socket wedged onto the end of the wheel nut to unscrew it.
Neither method is ideal and both render the wheel nut useless afterwards but if you've lost the key and the info required to get another one its the only way really.