Welcome to the forum. The real issue is that time is not on our side.
I purchased my first Mustang in 1976. I was a '64-1/2 Coupe, D code with a 4 speed. I paid $550. My second Mustang was purchased in 1978, it was a 1968 "S" code coupe, I paid $1500. At that time, those cars were 10 years old, and every junkyard was full of Mustang and Cougar parts. I used to fall over toploaders and 9" rear ends when scouring for parts in the local scrap yard.
So, roll forward to 2022. That '64-1/2 "D" code 4 speed coupe is not priceless, but rare as hens teeth. Today, that $550 car is worth 40-50 times what I paid for it back then. So are the parts. A good way to get extra parts that you might need is to buy whole parts cars. Take what you need, sell it to someone who needs the rest.
For mechanical stuff, find a large show like at Hershey or Carlisle Ford Weekend, where there are literally acres of parts to be had, sold by other enthusiasts that know what you are looking for, and usually, charge a fair price.
Or, ask someone on this forum. I personally have hoarded a full storage building of Mustang parts, to the point where I don't even know what I have in there. So, just ask, and you will be surprised at what response you will get. The folks on this forum are excellent people, honest, and will find you what you need if they have it.
Good Luck!