Sometimes, no download exists for a weird metric-to-imperial adapter or a vintage valve. You must build it.
If you cannot find what you need from Autodesk or manufacturers, forums and community sites are your next stop. The most common search results for "AutoCAD Plant 3D catalog download" lead to these sites.
Autodesk does not host a massive public repository of every manufacturer, but they provide a solid foundation.
Once you have downloaded your desired catalog (let’s call it My_Industrial_Catalog.pcat), you need to install it correctly. Many users fail here because they simply copy files to a desktop.