There are several steps required to getting devices into NetBox itself. The first has already been covered, setting up a Site for the device to reside at. This is going to be the table of contents of sorts for the upcoming posts that revolve around setting up a device.
|1. Site Setup|
|2. Manufacturer Setup|
|3. Platform Setup|
|4. Device Types|
|5. Device Roles|
|6. Adding devices|
Already covered here. Take a look there for getting your sites into NetBox via code specifically. To me, site information should not be sourced from an IT tool. This should be another database or tool that maintains information about your sites.
The manufacturers are just who makes the hardware. There may be a CMDB source available that would have this information. If that is the source of where you can buy gear, get it from here. Otherwise this demonstration is adding the information from a YAML file.