These are the most popular "classic" sites. They usually provide photos of handwritten solutions so you can see the process, not just the answer.
Don't just copy! Use the GDZ to after you've tried it yourself. It’s the best way to actually learn the rules and ace your next test. Common Authors to search for: gdz po russkomu iazyku klass prosveshchenie
Ladyzhenskaya (Ладыженская), Trostentsova (Тростенцова). These are the most popular "classic" sites