Most decent suppliers should be able to provide that for you. You could either order a custom made unit, or buy a stock unit and cut the shape you need out of it. The custom route is more expensive, because a small strip like that is likely to cost the same as a full topper. The stock route is much cheaper, because you could probably cut half-a-dozen strips out of a single stock unit. The problem is that the stock unit will only have one graduated hairline, and a graduated hairline is a very important element in getting a natural look. The second strip you cut out will not have that, so it will not look as natural unless you recreate the graduated effect by plucking out some of the hairs.
All in all, if this is your first system I would go the custom route, at least for the first one, unless money is a very big issue for you.
Noah