Try looking on popular reading websites for kids. Sometimes, libraries' online resources also have it available. You could also check educational sites focused on children's stories.
I'm not sure if you can find it for free legally. You might try some public domain book websites, but be cautious as many free sources could be unauthorized.
It's not a true story. 'Where the Wild Things Are' is a wonderful tale made up to spark the imagination of readers and viewers. There's no factual basis to it.