You can get the latest version of Bootstrap here. As you have correctly stated, Bootstrap is open source and as such is free to use subject to the MIT License.
DMXZone have created a Bootstrap extension for users of Dreamweaver. The initial extension, which is for Bootstrap 2x, has very graciously been supplied free of charge.
If you wish to use the latest version, you can do as suggested by going to the Bootstrap site. If you wish to acquire the extension for Dreamweaver, you will need to purchase it from DMXZone. A serious web developer will choose the latter.