gistfile1.m

int desiredRow = whatever;
    int currentRow = 0;
    for (int section = 0; section < [self.collectionView numberOfSections]; section++) {
        for (int row = 0; row < [self.collectionView numberOfItemsInSection:section]; row++) {
            if (currentRow == desiredRow) {
                indexPath = [NSIndexPath indexPathForRow:row inSection:section];
            }
            
            currentRow++;
        }
    }