All I can say is to read This
I watched the Anime when I was younger he finds it on route 24 when it is sat on a rock waiting for what it thought was its loyal trainer (he has abonded it). Ash saves its life but it runs off again. Then Team Rocket attack Ash and co. They catch pikachu in a rubber ballon bazooka but then Charmander turns up and monsters Team Rocket. Charmanders trainer turns up after seeing the battle and tries to get Charmander back put Charmander joins Ash.
The series that this episode (Charmander the stray) features in is called Poke Friends!
PS. I still have all the volumes on tape ;D
I have got the correct answer.Ash had caught his in Route 24.Source
Check the details on the right side of the page.There is a charizard picture,below it is its details.You should be able to see where it was caught.