Webinator may be obtained from http://www.thunderstone.com/texis/site/pages/webinator.html . There you may review the different versions that provide varying size limits and levels of support. Then, you may download the free version or order one of the paid versions.
Follow the instructions on the web site to acquire the package for your operating system. After registering for the free version, you will be given a URL to a compressed tar file for Unix versions or to an executable .msi program for the Windows version, and this will contain binaries for your specified operating system.