So basically you get the ROMs first and then try the emulators to find the one that makes them work right?
No, that only applies to ROMs which you buy, for which you don't know the Romset version. For other methods, please read the doc page that was posted a few times already in this topic.
But how can I purchase ROMs with a license? Nothing can be found.
There's a long topic over here with options on where and how to buy games, including arcade games.