Basically, any HTML based site would work inside PageCarton seamlessly. Any site that offers a downloadable archive of HTML templates are sites to look out for. Here are sites that provides free templates that works for building PageCarton Themes.
Don't forget that ready-made themes are on PageCarton Themes Repository - https://themes.pagecarton.org/
Other sites that have templates that fits into PageCarton are below.
The sites listed above have been tested as at the time this article was written. Feel free to report if any of the links are no longer valid or if there are other sites you want us to include here.