It's mainly about the daily lives and relationships of a group of teenage boys. The story focuses on their friendships, emotions, and experiences growing up.
Online manga platforms are a good place to start. Some well - known ones might have the '340 days manga side story'. However, make sure the platform is legal and authorized to distribute the manga. You can also check the official website of the manga's publisher, as they may provide information on where to get the side story, whether it's in digital or physical form.
You can try some popular manga reading websites like MangaDex or KissManga. They often have a wide range of manga available, including 'A Love in 100 Days'.