if you dont have the game, buying the game lol i dont think you can pay for an emu rom if you mean using the disk with the emu i think thats not a thing but idk

The only way to legally get a rom of the game is to own a modified 3ds and a physical copy of the game. There's also a method for ds that involves transferring it over the internet but I kinda doubt that it still works. You'd probably be better off downloading a rom online or playing the game on official hardware