FindAssetsByType

 public static List<T> FindAssetsByType<T>() where T : Object
    {
        List<T> assets = new List<T>();
        string[] guids = AssetDatabase.FindAssets(string.Format("t:{0}", typeof(T)));
        for( int i = 0; i < guids.Length; i++ )
        {
            string assetPath = AssetDatabase.GUIDToAssetPath( guids[i] );
            T asset = AssetDatabase.LoadAssetAtPath<T>( assetPath );
            if( asset != null )
            {
                assets.Add(asset);
            }
        }
        return assets;
    }