I want to own the rights to a song to put it in a creative work. The creators of that song live in a very tiny country, and have not registered copyright in the US. What do I do?

Location: California, US

The answer might just be: "You can use it freely if they haven't registered it with the US Copyright Office", which would be nice, though I would still like to pay them.

I am not asking them directly yet because I am not a lawyer and I don't want to be given a deal that either doesn't give me what I need, or is otherwise unfair. I would like a solution that isn't illegal anywhere, and I would like to compensate them fairly.

I would also like to have the rights to make derivative works with their song to put in the product I am making. I'm not sure if this changes my question.

I don't even know where to start. I've never bought the license for music domestically, much less abroad. I'm completely out of my depth. Anything (even pointers on what to look up) helps.