Sorry to learn of your difficulty.
I can't tell you what to do in this circumstance. Finding this key being offered for sale would most likely involve luck. You could try your Caddy dealer, but I assume you already have done that, and having done so know better than we do if this item remains available from Cadillac.
So as I said, I can't tell you what to do. I can, and I will, tell you what I would do:
I would make a key.
Of course that's easy for me to say, because I have my own (small) machine shop. I would probably fab up the key out of brass or nylon, something soft.
Assuming you have no shop, and most folks do not have a shop at home, you can take the problem to a small local machinest in your area. I would approach a one or two man shop - nothing larger. But a shop having the basics, a Bridgeport machine and perhaps a lathe
If you have any mechanical ability at all, try to sketch up a drawing of the key from memory, something you can show the machinest.. It does not have to be fancy. Anything is better than nothing. And drive to the machinest's place in your Cat so he can examine the location into which the key must fit. A decent machinest should be able to fab something up for you without undue trouble.
Good luck!
