How to find one private key from the list of 15 million public keys for the sake of experimentation?
That's how crypto works where public key cannot retrieve private key. If you were able to retrieve the private key with public key then bitcoin would be at high risk.

The only way is to Brute Force (Nearly impossible) or Social Engineering (Only if you know who owns the key) or Somewhere that stores leaked private key otherwise, you just can't obtain it.