You may contact PTC sales team for the same extension.
Alternatively, below article discussed about invoking Salesforce functionalities through REST API.
If you are not getting Salesforce extension, I believe you can start developing your own few REST API methods to call Salesforce functions.
If you need to connect with Salesforce, you can use REST API to perform any CRUD operation on the Salesforce side.
You can do it in two ways:
1. Using Thingworx services you can send any basic HTTP request.
2. Creating Thingworx extension - this is more flexible way, so I'll recommend this one.
Hope it helps, if you need any more assistance, don't hesitate to ask.
I ran into the same issues regarding Salesforce. The ThingWorx Salesforce connector no longer exists. I wrote a short step-by-step guide that walks you through authenticating, adding a record, finding a record, and modifying records: