This page contains an overview of the available options:
(under “Buying Books”). My personal favourite is BookWalker, the app is not amazing but fair to use imho. Just be aware that those books are usually tied to the app of the store, you cannot download them as PDF or something.
(If you really hate that, I think there are some threads on the forums that talk about getting books without this coupling, but I’ve never tried out any of those solutions. Maybe somebody else can advise in that case?)