EditorWindow what can apply one font to all of components that uses font in the currently opened scene and assets.

FontApplier.cs

using System.Collections.Generic;
using UnityEditor;
using UnityEngine;
using UnityEngine.UI;

public class FontApplier : EditorWindow
{
    private static FontApplier assetReferenceFinder = null;
    private Vector2 scrollPosition;
    private List<Object> references = new List<Object>();

    private Font fontField = null;

    [MenuItem("Font Applier/Find and Apply Font", false, 0)]
    private static void FindReferences()
    {
        assetReferenceFinder = GetWindow<FontApplier>();
    }

    private static void Find()
    {
        var textComponents = UnityEngine.Resources.FindObjectsOfTypeAll<Text>();

        var textMeshComponents = UnityEngine.Resources.FindObjectsOfTypeAll<TextMesh>();

        assetReferenceFinder.references.Clear();

        if (textComponents != null)
        {
            foreach (var text in textComponents)
            {
                assetReferenceFinder.references.Add(text);
            }
        }

        if (textMeshComponents != null)
        {
            foreach (var textMesh in textMeshComponents)
            {
                assetReferenceFinder.references.Add(textMesh);
            }
        }
    }

    private void ApplyFont()
    {
        if (fontField == null)
        {
            fontField = Selection.activeObject as Font ?? UnityEngine.Resources.GetBuiltinResource<Font>("Arial.ttf");
        }

        if (references == null)
        {
            return;
        }

        references.ForEach(obj =>
        {
            if (obj is Text)
            {
                (obj as Text).font = fontField;
            }
            else if (obj is TextMesh)
            {
                (obj as TextMesh).font = fontField;
            }
        });
    }

    private void OnGUI()
    {
        scrollPosition = EditorGUILayout.BeginScrollView(scrollPosition);

        fontField = EditorGUILayout.ObjectField(fontField, typeof(Font), false) as Font;

        if (GUILayout.Button("Find Components"))
        {
            Find();
        }

        if (GUILayout.Button("Apply this font"))
        {
            ApplyFont();
        }

        foreach (var reference in references)
        {
            EditorGUILayout.ObjectField(EditorUtility.IsPersistent(reference) ? "Asset: " : "Scene Object: ", reference, typeof(Object), true);
        }
        EditorGUILayout.EndScrollView();
    }
}