The Darmanitan line is exclusive to Sword, so if you have Shield, you'll need to trade. If you have Sword, you can only get Darmanitan with Zen Mode through raids. HA Darmanitan is available through common raids (red beam) in den 18, den 20, and den 23, and through rare raids (purple beam) in den 55. Darmanitan's spawn rate is highest in one of the common dens, so I would recommend trying to find one there. Images for where these dens can be found are in the links.