You can buy the Fire Blast TR at one of the Wild Area Watt vendors, though their selection changes from time to time, so you'll have to check all of them and perhaps check them every so often if it doesn't show up at first. Might also be a chance of getting it from a Fire type max raid battle too.
And Charizard can learn Fire Blast yes
“No, I... I have to be strong. Everyone expects me to."