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.
DLC update: Darmanintan is still a Sword exclusive, but it can now also be found in the Crown Tundra in den 168 as a common raid and den 169 as a rare raid. Additionally, den 169 is the only den in the game that Darmanitan has a 100% chance of having its HA for.