How do I get more robotic parts?

Solution 1:

Robot parts can be looted off of other robots you destroy, or they can be crafted just like weapon mods or armor mods through finding junk and creating it. Crafting the parts requires the Robot Workbench, which you can build after the Mechanical Menace quest.

Its similar to how if you have junk in your workbench at a settlement. When you go to modify the robots at the robot workbench, any mods you can build will be available for you.

A complete list of all the parts you can find/make can be found here at the Wiki (its rather long to list here).

Solution 2:

Robotic parts can be made as you defeat and find more advanced bots. Encountering a robot with a certain part, defeating it, and looting said part unlocks the blueprint/schematic for that part.