PrefabPalette.cs

// The returned Vector2 is the updated scroll position of the scroll area
// scrollPos is the current scroll position
// objects is the list of prefabs to draw in the palette
// selectionCheck is a lambda to check if a given object is the currently selected one, e.g. (go) => go == _currentSelection
// setSelection is a lambda to set the current selection, e.g. (go) => _currentSelection = go

Vector2 DrawScrollingPalette(Vector2 scrollPos, List<GameObject> objects, Func<GameObject, bool> selectionCheck, Action<GameObject> setSelection)
{
    var scope = new EditorGUILayout.ScrollViewScope(scrollPos, false, true, GUILayout.Height(300));
    var needsEnd = false;
    using (scope)
    {
        // we are currently hardcoded to have rows of 4, to change we'd need to update where we check this index for 0 & 3 to be
        // whatever we want
        var idx = 0;
        foreach (var obj in objects)
        {
            if (idx == 0)
            {
                GUILayout.BeginHorizontal();
                needsEnd = true;
            }
            // This is the call to actually get the preview texture. If you have lots of objects you might need to call
            // AssetPreview.SetPreviewTextureCacheSize() and set a higher value
            var preview = AssetPreview.GetAssetPreview(obj);
            var rect = GUILayoutUtility.GetRect(60, 60);
            if (Event.current.type == EventType.MouseDown)
            {
                var pos = Event.current.mousePosition;
                if (rect.Contains(pos))
                {
                    setSelection(obj);
                    Event.current.Use();
                }
            }

            if (preview != null)
            {
                EditorGUI.DrawPreviewTexture(rect, preview);
            }
            if (selectionCheck(obj))
            {
                EditorGUI.DrawRect(rect, new Color(1, 1, 1, 0.2f));
            }

            // if we're at the end of the row, close it up
            if (idx == 3)
            {
                GUILayout.EndHorizontal();
                needsEnd = false;
            }
            idx = (idx + 1) % 4;
        }
        // if we ended with a partial row, fill it with flexible space and close it out
        if (needsEnd)
        {
            GUILayout.FlexibleSpace();
            GUILayout.EndHorizontal();
        }

    }
    return scope.scrollPosition;
}