Is it possible to buy a legal third-party MacBook (Pro) charger?
I have long-term professional expertise regarding electrical safety.
Legal power supplies have extensive and expensive certification and testing and manufacturing controls to ensure the user's safety.
Some of the most safety sensitive aspects are:  


Isolation Voltage - How high a voltage may exist between the computer electronics and the mains wiring without a breakdown occurring.
Leakage Current - How much current can flow through the power supply (and ultimately you) if you make contact with a grounded contact or even a live wire while you are contacting the computer electronics.
Fire or burns


I will not claim that the "knock-off" supplies have NOT had the testing, but you decide for yourself if you think that Chinese manufactured supplies, even those bearing CE, UL, or CSA marks have actually passed or even been tested with the ultra-expensive tests and whether the manufacturing operation is well-controlled enough to guarantee that the product has not been modified.
How much would you spend to ensure that you or your child is not electrocuted?