A person or organization that provides support, often financial, political, or moral.
彼はそのプロジェクトの熱心な支援者だ。
He is an enthusiastic supporter of the project.
選挙では多くの支援者が集まった。
Many supporters gathered for the election.
支援者 is a general supporter who may give help of any kind, while スポンサー specifically implies financial backing linked to advertising or sponsorship deals.
Compound of 支援 (support) + 者 (person).