For data augmentation you can find something at:

  
      

      TensorFlow
  

  
    

Data augmentation  |  TensorFlow Core 1



  

  
    
    
  

  


I think that you can try also to create syntetic images or collect more real data.
With synthetic images, real data and augmentation you need to try to cover your representative test distribution  (plate type, numbers-letters, lighting, shadows, plate background, point of view etc…)