Thorium Reader is compatible with all retailers that sell EPUB or PDF files that are either DRM protected using Readium LCP or DRM-free.
If you're looking specifically for DRM-free ebooks, we recommend using Libreture's list of DRM-free bookshops.
If you're using Thorium Reader on iOS, please refer to our dedicated page for catalogs to learn how you can add these websites to our app.