If you own the game legally on Steam, do not hunt for manual downloads first. Steam has the official zone files.
Plutonium is the most popular third-party client for classic CoD games. It has a built-in automatic zone file downloader.
The zone folder acts as a library for the game’s level-specific data. Each file (typically with a .ff extension) contains the geometry, textures, and sounds required to load a particular mission or multiplayer map.