Not 100% sure but I believe they are random drops on the final boss in the weekly.
It doesn’t have to be weekly. You can do them on 390 weather it is weekly and you can still get them. You’ll just have a higher chance on weekly cuz of knuckles of eao and if you do challenge.
Thanks for the clarification, now he has a definite answer and can start grinding for the sparrows from KF and WotM.