There is a method to get a cell by
As of iOS 9, you can use
-[UICollectionView supplementaryViewForElementKind:atIndexPath:] to get a supplementary view by index path.
Your best bet is to make your own dictionary mapping index paths to supplementary views. In your
collectionView:viewForSupplementaryElementOfKind:atIndexPath: method, put the view into the dictionary before returning it. In your
collectionView:didEndDisplayingSupplementaryView:forElementOfKind:atIndexPath:, remove the view from the dictionary.