Use these: D’Addario EJ45 Pro-Arte Nylon Classical Guitar Strings, Normal Tension

They’re available everywhere including Amazon: https://www.amazon.com/DAddario-EJ45-Pro-Arte-Classical-Strings/dp/B000EEL6J6

Buy two and keep an extra pack in your case for when you bust a string. Then you can take the string out of the extra pack, get your guitar playing and order another string for the future.

Here are instructions on changing the strings: https://tomasmichaud.com/how-to-change-strings-on-a-classical-guitar/