This site is the official Titanium SDK downloads registry. Here you'll find General Availability (GA) releases, Release Candidates (RC), Beta releases, and Continuous Integration (CI) builds.
The easiest way to install these SDKs is to use the Titanium CLI. You may install as many SDK versions as you'd like.
# Step 1: Install the Titanium CLI
>npm i -g titanium
# Step 2: Install the latest Titanium SDK
>ti sdk install 12.1.2.GA --default