Dynamically set the sprite of a button in loop

Hi, I am working on Unity 4.7 project and have 15 UI buttons on my scene (named Button1, Button2, ...) and want to dynamically set the sprites or textures to it. I did it on next way:

 public Button[] buttonsFlags = new Button[15];
 
 ...
 
 public class TextureLoader
 {
     public Sprite[] textures;
     public void LoadTextures(string pgNumber)
     {
         string path = "Flags/page" + pgNumber;
         textures = Resources.LoadAll<Sprite> (path);
     }
 }
 
 public void FillTheFlagsPage(string pgNumber)
     {
         textureLoader.LoadTextures(pgNumber);
 
         for (int i = 1; i <= textureLoader.textures.Length; i++) 
         {
             string btnName = "Button" + i.ToString ();
             buttonsFlags[i] = GameObject.Find (btnName).GetComponent<Button>();
             //buttonsFlags[i].GetComponent<Renderer>().material.mainTexture = textureLoader.textures[i];
             buttonsFlags[i].image.sprite = textureLoader.textures[i];
         }
     }

But it seems something is wrong. I get NullReferenceException: Object reference not set to an instance of an object on line buttonsFlags[i] = GameObject.Find (btnName).GetComponent(); Please help me to find right way to do this.