Tripal v0.3.1b was originally housed at sourceforge.net. You can retrieve the downloadable archive package for Tripal v0.3.1b by following this link:
Download Tripal v0.3.1b Virtual Machine
Also available is a VirtualBox virtual machine with Chado, Drupal and Tripal pre-installed, with the database pre-populated with the Sequence and Gene Ontologies. This virtual machine can be used for testing Tripal with your own data without worrying about the installation. Download the virtual machine here. Use the VM tutorial for testing with a demo data set.