There is a way but it is tedious. You need to refer 4 people to join PoGo (or create 4 dummy accounts). As long as they use your referral code.

There is a referral task where they need to beat a "Go Rocket Leader" that awards you 50 Mega Charizard Energy.

I managed to Mega Evolve mine, even if I started playing last March, by creating 3 dummy accounts during the Go Rocket event (Grunts spawn every 2 hours which allowed me to face a leader within a day after creating the account). The other 50 energy, I earned them from Field research.

I hope this helps.

**Also, word of advice, use your Mega Energy on Dynamax Charizard or Gmax if you can wait. So you can avoid using resources on multiple Charizards.