Waves frequently runs promotions where you can choose a free plugin (including Vocal Rider) when spending over a certain amount (typically $50 or $80) on other tools.
A classic freeware VST (Windows) that handles vocal leveling. It is available via KVR Audio and is known for its simplicity and effectiveness.
While the original Waves plugin is a paid product, there are several legitimate ways to get it at no cost or use highly effective free "clones." Official Ways to Get Vocal Rider for Free