How do I file for divorce if we live in different countries?

You don't say where you now reside. In most parts of the world, you can seek a divorce if one of the spouses has lived in the country, or a district within it, for a given period of time. You should bend every effort to find out where your husband is, since you will need to 'serve' himi.e. have the papers officially delivered to him. Find a skilled family lawyer in your locality: it's almost always worth the effort. American Embassies and consulates often have lists of English-speaking local lawyers.