I think Comixology is a great option. It offers high-quality scans and has a user-friendly interface. Also, it often has exclusive content and updates regularly.
I think specialized manga apps such as Tachiyomi and Manga Rock are great. They often have updated content and customizable reading settings. However, make sure to use legal and authorized sources to support the creators.
Some indie manga creators post on social media platforms like Instagram or Tumblr. But keep in mind, the reach might vary. You could also consider creating your own website or blog if you want full control over the presentation and monetization.
Online platforms like Amazon are great. They have a vast selection of both manga and light novels. You can find popular titles as well as some rare ones. The prices are often competitive and they offer convenient shipping options, especially if you have Amazon Prime which can get your items to you quickly.