If you work in Revit and constantly looking for good content, one source is Autodesk® Seek. You can find everything from generic to manufacturers models and even find AutoCAD content. Both Revit and AutoCAD have a search feature within the software to use Autodesk® Seek but I hardly used it due to it not being very visible. Most of the time, I just went to the website because I found it easier.
Now there is a Autodesk® Seek Add-in for Revit, which in my opinion, is a great improvement. Here is a link to review and download it.