Dead Space

The weapon upgrades in Dead Space can be purchased from the Store (normally for around 10,000-12,000 credits) and found around the world, in side rooms, on the main path, or in rooms that you’ll need to backtrack to with security clearance.

Each weapon has three upgrades available to find, meaning that you’ll need to collect a total of 21 weapon upgrades for this achievement.

One important detail is that weapon upgrades are physical items that take up room in your inventory, and won’t be installed into your weapon until you reach a bench with them in your inventory. Because of this, if you accidentally drop them or sell them, you won’t be able to get them again until a New Game+ run, which is less than ideal.
Reaching a bench with a weapon upgrade in your inventory will see it installed in your weapon automatically - you don’t need to actually use your nodes to activate the weapon upgrade in order to get this achievement, although you’ll end up doing that anyway in order to unlock the Maxed Out (150G) achievement.

Plasma Cutter

• Cartridge Rack - Ishimura Clinic, Medical, Floor 4 (Chapter 2 onwards)
• Heat Accumulator - Purchased from Store (Chapter 3 onwards)
• Weighted Blades - Maintenance Locker Room, Communications Hub, Floor 2 (Chapter 8 onwards)

Pulse Rifle

• Kinetic Autoloader - Purchased from Store (Chapter 3 onwards)
• PCSI Custom Magazine - Electrical Systems Storage, Bridge, Floor 1 (Chapter 4 onwards) - This room is locked, but can be accessed by shooting the electrical lock inside the room through a broken window.
• High Yield Grenades - Water Purification Storage, Bridge, Floor 5 (Requires Master Security Override, see Full Clearance (30G))

Ripper

• Ricochet Tracer - Purchased from Store (Chapter 3 onwards)
• Angled Launcher - B. Andonov Flow Supervisor, Hydroponics, Floor 1 (Chapter 6 onwards)
• Carbon Fiber Blades - Machine Shop, Engineering, Floor 4, in a chest marked with a star on your map (Requires Master Security Override, see Full Clearance (30G))

Flamethrower

• Gellified Hydrazine - Purchased from Store (Chapter 3 onwards)
• Macrolitre Fuel Tank - Equipment Maintenance Bay Utility Room, Mining, Floor 1 (Chapter 7 onwards)
• High-Pressure Nozzle - Deluxe Shift Bunks, Crew Quarters, Floor 3 (Chapter 10 onwards)

Contact Beam

• Supersymmetry Tether - Purchased from Store (Chapter 4 onwards)
• Diffraction Module - Tools Storage, Mining, Floor 6 (Requires Master Security Override, see Full Clearance (30G))
• Portable Heliotron - Bioprosthetics Locker Room, Medical, Floor 1 (Chapter 7 onwards)

Line Gun

• Ionized Capacitor - Purchased from Store (Chapter 5 onwards)
• Precision Lasers - Hydroponics, Floor 1, in a chest marked with a star on your map (Requires Master Security Override, see Full Clearance (30G))
• Photon Energizer - Lab Storage, Medical, Floor 4 (Chapter 7 onwards)

Force Gun

• Subsonic Oscillator - Purchased from Store (Chapter 6 onwards)
• Gravitic Amplifier - Guest Consultation Suite Back Room, Crew Quarters, Floor 3 (Chapter 10 onwards)
• Suspension Module - Cargo Bay, Hangar Cargo Tram Control, Floor 1 (Requires Master Security Override, see Full Clearance (30G))