Im starting to think you have my old probe...lol
I had the same problem with my probe gt, and was why i got rid of it..
Your gonna have to get a spark plug re-thread kit, and just rethread it. None of the auto zone, advance, oreilly in our area, will carry the correct size.. atleast they didnt when i had to rethread a few weeks ago. I had to go to Ace hardware in Conyers, and got everything i needed.
But probe/mx6 gts are very well known to strip the spark plug threads. You may be able to find what you need at advance/oreilly/napa/AZ on covington.. but ace definatly has everything you need, they close at 6...get moving!
Another thing, i still have my rethread kit here at the house, and heli coils, i may have the size you need, but i do still have the spark plug rethreader and tap kit etc... if u want to try it out? im right around the corner!