Existing Layout Templates
There a handful of layout templates (or layout template systems) available for Backdrop, with more being added regularly. Here are several resources to find a layout template system for your site:
The Backdrop Layout templates page has stable releases.
The official distribution channel for layout templates is the Backdrop layout templates page on backdropcms.org. This page lists all layout templates that have an official release (or pre-release).
The Github backdrop-contrib pepository contains all projects.
You can find all of the above, plus other modules, themes, and layout templates that are still in development in the Backdrop Contrib Github repository. Projects that haven’t been released yet are likely not ready for production use. If you find something that you would like to use, please test it in a safe enviornment (like one of our demo sandbox sites). If it looks ready to use, please drop a note to the developer expressing your interest (or possibly your willingness to help bring it to production).
Developing Layout Templates
If you can’t find a layout template (or layout template system) that suites your needs, there is always the option of creating them yourself.
Layout templates in code.
You may also be able create your own layout templates with some CSS and HML knowledge. Documentation on developing layout templates can be found here.
Layout templates as configuration
Backdrop core includes the ability to create custom layout templates in the admin interface by defining rows, and columns to hold regions. These new layout templates are known as "flexible layout templates."