input: Write a python function to get an item by its ID from a list of items and return it as a JSON string.
  assert get_item_by_id(1) == '{"id": 1, "description": "A book", "price": 10.99}'
  assert get_item_by_id(2) == '{"id": 2, "description": "A pencil", "price": 0.99}'
  assert get_item_by_id(4) == 'Item not found'
output: import json
  
  def get_item_by_id(item_id):
      items = [
          {"id": 1, "description": "A book", "price": 10.99},
          {"id": 2, "description": "A pencil", "price": 0.99},
          {"id": 3, "description": "A notebook", "price": 2.99}
      ]
      
      for item in items:
          if item["id"] == item_id:
              return json.dumps(item)
      
      return "Item not found"