I think Amazon Kindle also has a wide range of comics. You could search there. Also, some official comic publishers' websites might have Heartstopper available for reading.
Well, actually, 'Heartstopper' is a graphic novel. It has been widely celebrated for its beautiful illustrations and engaging story that follows the lives of its characters, especially in the context of LGBTQ+ youth experiences. The format of a graphic novel allows for a unique blend of visual and written storytelling that has made 'Heartstopper' so beloved.