How did she find Ripley's house?

If you go to Highsmith's text Ripley and his wife (He's married in the novels) live in Villeperce, France at Belle Ombre. Most people in town could tell you where he lived. For the film it's presumed the same.

It's a good question. I wondered the same thing. But the film operates as an allegory at its deepest level. How she found his house is artifice. What really matters is that Jonathan sold his soul to America (Dennis Hopper) to save Germany (his wife). When characters represent metaphors their actions transcend their human qualities.

If the film was built on realism your question would be relevant. But it's not.

If Ripley wasn't listed in the phone book then Marianne could have gotten the address from the auction house where she worked and he was a client.