See these related discusions:

https://www.researchgate.net/post/How_to_generate_an_DOI_for_publications

https://www.researchgate.net/post/Getting_DOI_for_individual_papers

https://www.researchgate.net/post/How_can_I_get_DOI_for_my_paper

https://www.researchgate.net/post/How_can_I_get_a_doi_for_an_article

https://www.researchgate.net/post/Adding_DOI_to_documents

https://www.researchgate.net/post/Is_there_special_app_to_generate_Doi

Dear Amunnadi Chukwudi Ani,

Refer the following link it help to you get all most all DOI registration agency:

https://www.doi.org/the-community/existing-registration-agencies/

Additionally you can also refer to the following link that explain how to get free DOI for your document:

https://ojs-services.com/journals-publication-indexing/how-to-get-doi-for-free/

Hope this will be useful.