How to get list of folders in sidebar?
You can't.  Finder's Sidebar is a location for shortcuts to be easily accessible by the user.

From Apple's Human Interface Design Guidelines:  


  the sidebar in a Finder window includes a list of frequently accessed locations and tag-based search shortcuts. The user can select one to access its contents or results in the main portion of the window.


Additionally, having a folder list is not practical in this element. Anything more than 2 levels of hierarchy (which is highly plausible in a folder listing) is discouraged.  In this instance, Apple states that a column view be used.


  In general, refrain from exposing more than two levels of hierarchy within a sidebar. In some cases, a second sidebar may be warranted
  when a data hierarchy is deeper than two levels. If your app requires
  the navigation of deeply nested objects, consider implementing a
  column view.


Bottom line; Apple discourages 3rd party developers from doing this, meaning Apple's Finder is definitely not going to show files like this.  

You can put a shortcut to a folder, but you won't get a list of folders.