Look, the whole legal manga ebook thing is a mess compared to Western comics, but it's getting better. First off, stuff like 'Spy x Family' or 'Chainsaw Man' you'll find on official apps like Shonen Jump or Manga Plus—those have subscription models that let you read offline, which is basically downloading. For older or less mainstream series, services like Kindle, Kobo, or Google Books usually have the widest selection to purchase per volume. I check if the publisher has an official digital storefront, like Yen Press or Seven Seas, because sometimes they run sales. It's slower and more expensive than fan scans, but at least the translation quality is consistent and the creators get a cut.
One annoying catch: regional licensing. A series might be available legally in ebook form on BookWalker in one country but not on Amazon in another, so you might need to try a few storefronts. I usually stick to the big platforms because their apps handle downloads reliably for reading on a tablet during commutes.