If your system specifically requires version 6.6.1 (e.g., due to OS limitations), Native Access should automatically offer the latest compatible version for your current OS. :
The official method to acquire this specific version is through the Native Instruments Support Page for legacy installers. kontakt 6.6.1 download
Kontakt 6.6.1 Download: Get the Latest Version of the Popular Sampling Instrument If your system specifically requires version 6
: A series of cinematic instruments.