Found this aftermarket TCI
https://www.aliexpress.com/item/Motorcycle-Digital-Electronic-Ignition-Racing-CDI-TCI-Box-Unit-ECU-For-Yamaha-XZ400-14X-XZ-400/32729210329.html?spm=2114.40010208.4.37.I34Ena (https://www.aliexpress.com/item/Motorcycle-Digital-Electronic-Ignition-Racing-CDI-TCI-Box-Unit-ECU-For-Yamaha-XZ400-14X-XZ-400/32729210329.html?spm=2114.40010208.4.37.I34Ena)
Generally I've found it fairly safe buying from Aliexpress.
Can't say what this is like so buy at your own risk (obviously). I can say first hand that the stock XZ400 TCI works fine in the 550 - if anything a bit better I think. It has a slightly different curve.