Do you get Luvdisc in fishing or by surfing in moon in order to retrieve some heart scales?
Route 9: Encountered by fishing.
Melemele Sea: Encountered by fishing.
Source, with full encounter rates.
You can find them by fishing at Route 9 or Melemele Sea.
By the way, if you want to teach flamethrower to your Charizard, then it's probably easier to use the TM than it is to obtain and use a heart scale.