easier:
https://www.turtlediary.com/game/plural-nouns.html
https://www.quia.com/rr/6516.html
http://eslgamesworld.com/members/games/ClassroomGames/Billionaire/Plurals%20Billionair%20Game/index.html
harder:
http://www.softschools.com/language_arts/la_games.jsp (choose "more complex")
https://www.turtlediary.com/game/singular-plural.html
choose your own difficulty level:
https://www.funbrain.com/plurals/
https://www.turtlediary.com/game/identifying-correct-plural-of-a-noun.html (choose 4th grade or higher)