I'm sure this isn't the smartest of all questions to ask but I have been rather curious. How do the owners of the vintage mustang keep them flying? Their old aircraft and all the parts supplies would have been depleted now wouldn't it?
For starters, most of the survivors don't get pushed very hard (excluding highly modified racers), with limits on manifold-pressure and top speed imposed by the owners. Also, out of the 15,000 or so built, only around 200 are still flyable; that could mean an awful lot of spare parts, even today. That said, cost is probably no object for the typical P-51 owner, so I reckon they could have just about anything they want fabricated. It also helps that the planes are registered as "experimental" by the FAA, which means they can be modified however the owner wants to remain airworthy