[rockguy]

www.3dcontentcentral.com will have some of these things.

Plants are a tough one and I'm not aware of any that are available for SolidWorks. Paul Salavador has an exmple of one www.zxys.com but it may not be usable?

If you use Maxwell Studio I believe you can bring in other objects besides the SolidWorks model and it should be easy to find the things you're looking for on the web in a file format readable by Maxwell Studio.