It is possible to obtain a Charmander easily in Pokémon Sword and Shield, but only after defeating the Master of Galar: Leon.
As you will have realized, this is a part of the End-Game. Leon being a fan of his Charizard, he left you a Charmander egg in his room!
How to get Charmander?
After becoming Master of Galar, return to Postwick (the first town) and go to Leon's house.
Upstairs, on the right, you will find Leon's room. On the ground, a Pokéball awaits you. It contains a level 5 Charmander.