Hit the demon bell, pop some green balloons and start at the Antechamber in the castle. Take out the four monks and rest if it is the first Owl, if it is the second owl y'all just have to fight your way through whatever is there depending on your world state. Don't give up farming, I spent 15 minutes once farming there an shot only one confetti but managed to farm nearly 50 within two hours on another segmented few goes but be sure thst it is the blue monks you are downing. Just remember to turn off the damn demon bell I the inventory. First owl or second owl? If it is the second owl and you're at very late game it is possible to buy confetti after a certain world change.
Everything in the above reply is my opinion, from my own perspective and not representative of reality outside of my own head!
-Android user, please be gentle with critique on my spelling.