You could check out your local library. Sometimes they have manga collections and might have Omori. Also, some bookstores might carry it or be able to order it for you.
You might find the Omori manga on dedicated manga apps such as Webtoon or Tappytoon. Also, some fan-run websites might have it, but be cautious as they might not have the proper authorization.
I don't have the exact name of the person who created the Omori manga at hand. It could be worth doing a search on manga databases or asking fellow manga enthusiasts for the answer.