There is only way to get it.
Stasis Rifle Guide
Stasis Rifle removed from Below Zero but you can still get that rifle by using console commands.
To activate Console you need to press F3 and you’ll see a checkbox ticked in the top left saying: “Disable Console”. Press F8 to get a mouse cursor and uncheck the box to activate the console.
Once the console is activated, press Enter to open the console and type “item stasisrifle” without quotes and press Enter again to activate the command, and then this rifle will be added to your inventory.
The rifle textures have been removed from the game, but it is still useful.
You can freeze all creatures with this rifle and you can kill all leviathans easily.