Found in Raven Rock on the right desk in Colonel Autumn's bedroom. After you pass the huge door from section 2B to 2C, when two Enclave soldiers come out of the right door, take the left one. Be sure to grab this before enter the War Room on your way to see President Eden, as you cannot return for it once you have already done so.
The Bobblehead seems to be carrying a weapon similar to the Alien Blaster.
If you missed the Bobblehead and play on PC, you can either use the console to spawn the Bobblehead next to you player.placeatme 3892f 1 or go back to Raven Rock by using the player.moveto 000918b0 command.
Obtaining Bobblehead Energy Weapons