A simple pattern for catching Camera.main in Unity.

CameraUtility.cs

using UnityEngine;

public static class CameraUtility
{
    public static Camera MainCamera
    {
        get
        {
            if (_mainCamera == null)
            {
                _mainCamera = Camera.main;
            }
            return _mainCamera;
        }
    }
    private static Camera _mainCamera;
}