AIf you are a good writer, then you may be able to create the article by yourself. And if you have the free time to spend to outreach other webmasters, then you can also find the hosts by yourself. In this case you do not need any guest post service. But you need to check if the time and effort spent will cost you more than using the services of a professional agency. Also, you should not expect the same level of results as those produced by an agency.