Quote From JoIs14:
Hello everyone, I want to visit my old referees to ask for the reference. Even though I am in another country doing the phd, the distance is not so far. So I can afford the ticket and time to go back to the country where I did my master. However, I am wondering if having a person-to-person meeting with them is appropriate. I don't want to bother their work. Look forward to every piece of advise and thought.
It's fine to just email them and ask them whether they will give you a reference... that is the usual practice.