The project sponsor is not a dedicated role in the organization, meaning no one goes and hires a project sponsor. The project sponsor is usually an executive in the company. For example, the project sponsor for a software project can be the IT director.
Now that I've explained that, it's clearly easy to understand that the person responsible for the project sponsor, or, in other words, the person that the project sponsor reports to is the one the latter reports to in the organizational hierarchy. So, if the project sponsor is an IT directory, and if the IT director reports directly to the Managing Director, then it's the Managing Director who's responsible for the project sponsor.