Vectorworks comes with a lot of nice furniture, in the Libraries > Objects-Imperial or Objects-Metric. But, what if you want something special, like a Ray and Charles Eames chair?
I recommend the sketchup warehouse (http://sketchup.google.com/3dwarehouse/).
Later versions of Vectorworks can import sketchup models, so you can download a chair from the warehouse and import these into a new file. Always import into a new file, that way you can edit the model and make it in to a symbol to add to your library.
The sketchup library is searchable, so you can search for the things you want. It is always being expanded, and the models are usually good.
Watch out for very large models, it will make your Vectorworks files large.