As an adult ESL instructor, I initially had a difficult time finding materials not geared toward children. There are many great online resources! A few of my go-to sites are below:
1. Allthingsgrammar.com / Allthingstopics.com (great for beginner to upper intermediate)
2. Headsupenglish.com (great for beginner to advanced--grammar, listening, and speaking)
3. Perfect-english-grammar.com (great for beginner to advanced)
Happy Teaching!