First, enable the "Level skip/select" code. Then:
Desert Ranger: Go back to the Serpent night club (the garage entrance). Push the yellow button, then retrieve the socket spanner. Pull the lever beside it. Then, barge into the door above the stairs. Get the Desert Ranger on the desk.
M-V9: After pulling the lever in the same area, go in the hole below the car. Use the socket spanner on the lever down here, then pull it to make one of the guards run out. Get rid of him and take his M-V9.
Dart SS: Go to Renne's Pawn Shop. Enter in the code by the keypad guarding what is inside. You should already know the code (it is in your notebook). Grab the Dart SS.
K2 Impactor: Do the same thing as done for the Dart SS, then grab it and run out or level skip.
Rigg 09: Already available after Von Croy's Apartment.
Viper SMG: Already available after Von Croy's Apartment.
Vector R35: Go to the Derelict Apartment Block. Play as done before. However, before you leave, grab the keys from the top level in the room (on floor and hanging just in case). Pull the switch and the get on the elevator. Go to the second floor. Jump the hole. Unlock the door. You will find it in the locker. You will not find the other Vector, since there is no other pistol.
V-Packer: Go the Tomb of Ancients. Complete the level until you get through the vent in the floor. Go straight then hang a right. Barge through the door. Reclaim the V-Packer and ammunition.
Mag Vega: Go back to Galleries Under Siege. Head up the stairs and get rid of the museum guard. Continue up until you find your first Agency soldier. Get rid of him and take his Mag Vega. You will not need to get ammunition in this level; you can just go to the Monstrum Crimescence and find some there.
Scorpion X: Complete the Monstrum Crimescence level and the reporter will hopefully give it to you.
Boran X: Start the Sanitarium level to get it.