I would check out
https://www.keebtalk.com/t/list-of-keyboard-retailers-shops-stores-vendors/
A quick glance of the list showed these vendors but there is probably more. Good luck cable making!
I made a few mistakes when I started cables IMO a few tips:
- Order a few extra connectors just in case
- Breakout boards and multimeter help to make sure you didn’t mess anything up… I
may haveburned out a num pad because I reversed 2 wires…