You'll need to collect extras of the items. For Mystery Goos, use my SUPER-SPIFFTASTICAL-TRICK-OF-ETERNAL-GODLY-GREATNESS-NESS-NESS!!!!

Go to Halloween Town

Go to Pumpkin Patch

Keep re-entering until Black Fungi appear

Hit big orange pumpkins

Throw pumpkins at Black Fungi


They will explode and drop 1000 munny and a mystery goo.