NCunlock comes to mind, but not sure they even accept unlocks as they dont have the xiaomi 15 ultra on their site.
Also they are super expensive with 50-80 usd.
From my understanding, Xiaomi are clamping down hard on unofficial BL unlocking "Services" like NCUnlock
Even last year with the 14 series it was quite difficult
Unlocking the CN variant for free is impossible I'm afraid
No, unless you are china resident and have level 5 without warnings in the community app and pass an Xiaomi exam. No go either using a paid 3rd party method.