Since there is NO "get_dynamic_sidebar" in the core, to return sidebar content instead of echoing out use the method below.

get-dynamic-sidebar.php

<?php

/**
 * Gets the sidebar content based on given sidebar ID.
 *
 * @link 	  https://forums.envato.com/t/get-dynamic-sidebar/76169/3?u=mypreview
 * @param  	string $id    ID of required registered sidebar.
 * @return 	string|html 	The content of sidebar returned with HTML markup without "echo".
 */
if ( ! function_exists( 'prefix_get_dynamic_sidebar' ) ) :
	function prefix_get_dynamic_sidebar( $id = NULL ) {

		// Bail out, if the ID is empty or missing.
		if ( empty( $id ) ) {
			return NULL;
		}
		// Bail out, if there is no sidebar registered with given ID.
		if ( ! is_active_sidebar( $id ) ) {
			return NULL;
		}

		$sidebar_id = esc_attr( $id );

		ob_start();
		dynamic_sidebar( $sidebar_id );
		$sidebar_contents = ob_get_clean();

		return $sidebar_contents;

	}
endif;

Nicely done!