I found this website useful when teaching students how to diagram sentences: https://www.english-grammar-revolution.com/english-grammar-exercise.html 

Each section contains some free content, and then to access the full content you have to buy the ebook.  I haven't bought the ebook, but I've still the freely available content to be very useful for providing exercises for my students, because it gives exercises and then answers.  

(It's part of a larger site: Grammar-Revolution: Grammar Through Sentence Diagramming, which has a lot of other useful material.)