You can get the Source SDK Base 2010 here.
What Is It?
The Source SDK Base 2010 is an unofficial SDK base of Valve's Source Engine. The branch is based of the Alien Swarm Solution, and features improvements seen in Portal 2. It's main focus is to be an alternative to The Source SDK Base 2013 as there are many issues with that engine branch when it comes to building a mod.
How To Install
The SDK2010 is set up like the other Source bases. Simply drag and drop the Source SDK Base 2010 folder under your ...steam/steamapps/common folder with your other games. The src folder can go anywhere you wish!