If you have access to Adobe Animate, the included AS3 components are pretty good. They were written by the GSkinner team. When I originally started building the components that would eventually become Feathers, I started out by extending
fl.core.UIComponent because it saved me some time.
You don't need to use Animate to compile your app, though. You can find the source code on your hard drive and add it to any AS3 compiler's source-path. If you do it this way, they won't have default skins, and you'll need to provide your own immediately. If you prefer to start with default skins, you can add all of the components to the library in Animate, and then right click and export a SWC. Add that SWC to the compiler's library-path.