The door key for a '68 is normally the same as the ignition. I presume yours is different for some reason?

Did you ever check to see if your key matched the Billing History code? If so, just take that code to a GM dealer or locksmith (not the corner hardware store) and have them make a key to that code. You have a 50/50 chance anyway, and it's worth the $2.50 gamble.

Instead of making spare keys, I made photocopies of my keys. A locksmith can compare the photocopy to the GM key chart and make one easily. Just note which blank is used.

If not, a locksmith can indeed make a key for your lock cylinder. You can call the locksmith to the car, or take the ignition switch to the locksmith. You could just take the cylinder, but you can't remove it without the key.

Don't feel bad, the key to my yellow car was lost hours before I first saw it, I had to call a locksmith to get it home. And you know the keys will show up...just after you've had one made.