slate tiles

Hi all, I bought a home in March 2023 and noticed recently after heavy rain that the front entrance to the property is allowing water to enter above the front door. From the photos attached, I have shown the front door and the square shaped entrance. I have attached photos from the outside...

I'm not sure where I can buy roof slates? If I get them would I need to cut to size? Is it better to just get a roofer to do it? It's only a ground floor annex, these things fall off from time to time so it may be a good skill to learn. Thanks for any help you can give me