You can get a thread repair kit for spark plugs or look for a kit called a Helicoil to repair the threads. The only reall problem when reparing this kind of issue is that the new threads will turn all the way in until they fall into the psiton area. So, I sugest going to a repair shop as you really have 2 issues. You need to find out what could have created the force that blew the plug out and stripped the threads because that is a tremendous amount of pressure and could be dangerous so ydon't work on the motor with it running until you find this out. Then you can get the threads repaired and that is genrally inexpensive. It is hard to say how much diagnosis of the reason for the pulg blowing out will cost but this is importnat to do first as it will happen again, most likely, and if you repair the threads when this happens it will take a larger amoutn of material with it making repair to the head impossible and you will have to get antoher one. So, first find out why the plug blew out of the engine. That is critical.