http://anthonyhalderman.com/english/compsup.htm

https://www.usingenglish.com/quizzes/45.html

http://www.softschools.com/quizzes/grammar/adjectives/quiz328.html

This site below has several additional links:

http://www.learnenglish-online.com/grammar/tests/superlativecomparative.html

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-12647.php

Short (10 questions) http://www.grammarbank.com/comparative-superlative-quiz.html