The Application:

<?xml version="1.0" encoding="utf-8"?>

<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
    layout="absolute"
    creationComplete="init()"
    xmlns:controls="com.mariosh.controls.*" viewSourceURL="srcview/index.html">

    <mx:Script>
        <![CDATA[
            import mx.collections.ArrayCollection;

            [Bindable]

            private var ac:ArrayCollection;

            private function init():void
            {
                ac = new ArrayCollection();
                for(var i:int=0; i<30; i++)
                {
                    var item:Object = new Object();
                    item.label = "item " + i;
                    ac.addItem(item);
                }
            }

        ]]>
    </mx:Script>
    <controls:BasicList
        x="10"
        y="10"
        width="100"
        height="200"
        dataProvider="{ac}"
        verticalScrollPolicy="off"/>
</mx:Application>

BasicList.as

package com.mariosh.controls

{
    import flash.events.Event;

    import mx.controls.List;

    public class BasicList extends List

    {
        private var scrollbar:ScrollBar;

        public function BasicList()

        {
            super();
        }

        override protected function createChildren():void

        {
            super.createChildren();

            if(!scrollbar)
            {

                scrollbar = new ScrollBar();
                scrollbar.addEventListener(Event.CHANGE, thumbChange);
                addChild(scrollbar);
            }

        }

        private function thumbChange(event:Event):void

        {
            var listIndex:int = scrollbar.scrollPercentage *(this.dataProvider.length - 9);
            this.verticalScrollPosition = listIndex;
        }

        override protected function commitProperties():void
        {
            super.commitProperties();

            scrollbar.width = 10;
            scrollbar.height = this.height;
            this.verticalScrollPosition = 10;
        }

        override protected function updateDisplayList(uW:Number, uH:Number):void

        {
            super.updateDisplayList(uW, uH);

            scrollbar.move(uW-scrollbar.width, 0);
        }

    }
}

ScrollBar.as

package com.mariosh.controls
{
    import flash.events.Event;
    import flash.events.MouseEvent;

    import mx.core.UIComponent;

    [Event(name="change", type="flash.events.Event")]

    public class ScrollBar extends UIComponent
    {
        private var track:Track;
        private var thumb:Thumb;

        private var _yOffset:Number;
        private var _yMin:Number = 0;
        private var _yMax:Number;

        public function ScrollBar()
        {
            super();
        }

        private function thumbDown(event:MouseEvent):void
        {
            stage.addEventListener(MouseEvent.MOUSE_MOVE, thumbMove);
            stage.addEventListener(MouseEvent.MOUSE_UP, thumbUp);
            _yOffset = this.mouseY - thumb.y;
        }

        private function thumbUp(event:MouseEvent):void
        {
            stage.removeEventListener(MouseEvent.MOUSE_MOVE, thumbMove);
            stage.removeEventListener(MouseEvent.MOUSE_UP, thumbUp);
        }

        private function thumbMove(event:MouseEvent):void
        {
            thumb.y = this.mouseY - _yOffset;
             if(thumb.y <= _yMin)

                thumb.y = 0;
            if(thumb.y >= _yMax)

                thumb.y = _yMax; 

            _sp = thumb.y / _yMax;

            dispatchEvent(new Event(Event.CHANGE));

            event.updateAfterEvent();
        }

        private var _sp:Number;
        public function get scrollPercentage():Number

        {
            return _sp;
        }

        //--------------------------------------------
        // override methods
        //--------------------------------------------
        override protected function createChildren():void

        {
            super.createChildren();

            if(!track)
            {

                track = new Track();
                track.width = 10;
                addChild(track);
            }

            if(!thumb)
            {
                thumb = new Thumb();
                thumb.width = 10;
                thumb.height = 40;
                thumb.addEventListener(MouseEvent.MOUSE_DOWN, thumbDown);
                addChild(thumb);
            }

        }

        override protected function commitProperties():void
        {

            super.commitProperties();

            track.height = this.height;
            _yMax = track.height - thumb.height;
        }

    }
}

Track.as

package com.mariosh.controls

{
    import mx.core.UIComponent;

    public class Track extends UIComponent

    {
        public function Track()
        {
            super();
        }

        override protected function updateDisplayList(uW:Number, uH:Number):void

        {
            super.updateDisplayList(uW, uH);

            graphics.clear();
            graphics.beginFill(0x333333);
            graphics.drawRect(0, 0, uW, uH);
            graphics.endFill();
        }

    }
}

Thumb.as

package com.mariosh.controls

{
    import mx.core.UIComponent;

    public class Thumb extends UIComponent

    {
        public function Thumb()
        {
            super();
        }

        override protected function updateDisplayList(uW:Number, uH:Number):void

        {
            super.updateDisplayList(uW, uH);

            graphics.clear();
            graphics.beginFill(0x666666);
            graphics.drawRect(0, 0, uW, uH);
            graphics.endFill();
        }

    }
}