You can get ALL Lvl 1 Implants (except PSI), Lvl 1 and 2 Eyes, Backbones 1, 2, 3 (and 4 for the Reflexbooster), Lvl 1 and 2 Hearts at either Crytons (NC) or Tech Angels (Dome and TH).
PSI-Implants 1 and 2 (and 3 for the PSI Controller) can be bought at the PSI-Smugglers (PSI Equipment) in Pepper Park and MB and the PSI Stores in the Dome.
Brain Implants Lvl 2 (except PSI) can be bought at a Smuggler in Point Red. Theres another one near the Dome afaik, i dont know the axact Location though.
ALL Lvl 3 Brain Implants (except for the Vehicle Interface 3 and PSI) can be bought from the Biotech-FSM.
Vehicle Interface 3 is a Next-Hacknet Item
All Lvl 3 PSI-Implants can be bought from the Crahn-FSM
The Marine CPU (for Tanks or Private Eyes) is a Twilight Guardian-Hacknet Item.
The Special Rigger Interface (for Dronerspys- or Private Eyes) is a Tangent- AND Tsunami Syndicate-Hacknet Item.
The Psicore CPU (for Monks) is a Crahn-Hacknet Item.
The Special Forces CPU (for Spys and Private Eyes) is a City Admin-Hacknet Item.
The Special Science CPU (also Spys and Private Eyes) is a Biotech Hacknet Item.