How do I get toolbar available items in CKEDITOR 5?

you can use console.log( Array.from( editor.ui.componentFactory.names() ) ); which will give you:

["undo", "redo", "bold", "italic", "blockQuote", "ckfinder", "imageTextAlternative", "imageUpload", "heading", "imageStyle:full", "imageStyle:side", "link", "numberedList", "bulletedList", "mediaEmbed", "insertTable", "tableColumn", "tableRow", "mergeTableCells"]

Example code you can use to list available toolbar

var editor = ClassicEditor
    .create(document.querySelector('#editor'), {
        toolbar: ['headings', 'bold', 'italic', 'link', 'bulletedList', 'numberedList'],
        heading: {
            options: [
                {modelElement: 'paragraph', title: 'Paragraph', class: 'ck-heading_paragraph'},
                {modelElement: 'heading1', viewElement: 'h1', title: 'Heading 1', class: 'ck-heading_heading1'},
                {modelElement: 'heading2', viewElement: 'h2', title: 'Heading 2', class: 'ck-heading_heading2'},
                {modelElement: 'heading', viewElement: 'h3', title: 'Heading 3', class: 'ck-heading_heading3'}
            ]
        }
    })
    .then(function (editor) {
        console.log(Array.from(editor.ui.componentFactory.names()));
    });

You can put this code right in the body of code samples which you can find e.g. in CKEditor 5 Build's Basic API guide. For example:

ClassicEditor
    .create( document.querySelector( '#editor' ) )
    .then( editor => {
        console.log( Array.from( editor.ui.componentFactory.names() ) );
    } )
    .catch( error => {
        console.error( error );
    } );

As @Szymon Cofalik mentioned in his answer – there's no single list of buttons which are available in all builds. CKEditor 5 builds may differ not only visually – they may also contain different plugins and hence different buttons. So, using that code snippet is the safest and future-proof solution.