Where can we find fan mail addresses for celebrities?

[derekcullen]

My little sister wants to send a letter to Greta Gerwig but I honestly have no idea how to do that outside of social media. There are a million different hits on google when I search but I don't know which are real and I would hate if my sister's mail got bounced back to her. To be clear I imagine it would go through her agent or studio or something, we're not looking for Greta's actual address.