Several options:
1. You can build one yourself if you really want to. That means learning a lot about Flash before you get something usable.
2. If you are using Flash MX, there is already a scrollbar component available from the component library. The drawback (sort of) is that this scrollbar adds quite a chunk to your final filesize, but it does look nice, and it is totally usable (draggable, etc.).
3. Also in the components library there is the scrollpane component. That looks even nicer. Again it adds to your filesize and it is a bit more difficult to set up than the scrollbar (a problem if you don't know Flash too well).
4. You can download one (another component) from this website:
http://www.flashcomponents.net/compo...m?nav=2&per=15
These are generally much smaller and are quite easy to use (though not much more so than the in-built scrollers).
5. You can go to any of the Flash help sites and download a movie that has a scrollbar in it.
It all depends on what you want: do you just want a scrollbar or do you want to build one from scratch?