The statues required to unlock the Solar Gun can be found at the following locations:

Frog: In Act 1: Middle East, Advent Palace garage, atop a vehicle.

Octopus: In Act 2: South America, in the Research Lab Boss room, upper right room during the human phase of the fight.

Raven: In Act 3: Europe, on the top floor of the human phase of the Boss fight.

Wolf: In Act 4: Shadow Moses, behind Snake's starting spot in the human phase of the Boss fight.

Mantis: In Act 5: Outer Heaven, near the northern-most door during the human phase of the fight.

--NOTE--
these will only be available if you defeated the boss NON-LETHALLY (mk II, mosin nagant, stun grenades, CQC, ect). If you killed the boss they will not be available.

--NOTE 2--
the solar gun causes enemies to drop weapons and does extreme damage to Vamp. To reload you must stand in sunshine and snake will point the gun up to the sky and shout SUNLIGHT!