The first step would be to get a stand alone emulator. There are quite a few for GBA, but I've had good luck with VBA, or visual boy advance. In my experience, as long as you have a computer less than 10 years old, it plays most games at a reasonable speed.
The next issue is finding the rom. We can't tell you where to find roms, but a Google search is usually pretty helpful. Just search for (game name) rom or something like that.
Edit: I hope I'm not breaking the rules even telling you that.
Edit 2: I hadn't even thought about if you were on mobile. On Android I use MyBoy. If you're on iPhone, as far as I know there's no emulators available without jailbreaking it. Also, I've never used an iPhone, so I don't have any recommendations.
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