A person or entity that supplies goods or services. Neutral, business-like term.
この会社は大手自動車メーカーの供給者です。
This company is a supplier for a major automaker.
供給者 is a more economic/formal term for someone who supplies goods or resources, while 提供者 is broader and can describe any kind of provider, including of information or services.
Compound of 供給 (supply) + 者 (person/entity).