It can only be obtained in Kanto, after you beat the Johto segment of the game. It's also extremely slow, and weak to very common types. If you want to use Magcargo, go for it, anything works ingame. Just know that it's worse than your other options.
You have the possibility of getting a Slugma Egg from Primo, but it takes a while to hatch, and it'll be annoying to level it up from Level 1. If you don't mind waiting, better Fire types like Arcanine and Magmar can be obtained a bit later in the game.
Hope I helped!