You first get the Surge Weapon form when you start the AWE DLC (A Dark Place) Which Unlocks as soon as you get the Polaris Mission. (just before the Ashtray Maze)
Just a Heads up for those of you who don't have the final blueprint to completely upgrade the Surge:
I was completely out of Untapped Potential at the time (boss fight reward item) so I was only able to create the gun and make the first upgrade by the end of the AWE DLC and the blueprint for the Final Surge Upgrade wasn't even showing.
It wasn't until I talked to Pope in Executive and completed the Mr Tommasi Sidequest and gained an Untapped Potential that I was able to complete it.