There are many sources of free models available on the internet. Most of them will not be Strata .s3d model files but they will have formats Strata can import like Collada (.dae), OBJ, VRML, STL, etc.
You can find a list of supported file formats here:
https://www.strata.com/design-3d-cx-8-for-mac/#features
There are models available on the StrataCafe:
http://www.stratacafe.com/models.asp
In addition, we sell some shapes libraries on our website: