Get the rectangular type of ND Grads, such as the Graduated Gray Cokin. On that lens I believe you can use either the A or the P series Cokin sizes. I'd recommend the P series only because there are more accessories and filters made for that size by other manufacturers. A series are smaller and less expensive though. Cokin's filters are relatively inexpensive, as are their basic holders and adapters. Later, if you wish, there are better quality, more expensive filters made to fit the P series size holders... from folks like Lee, Tiffen, Singh Ray and more. There are also handy accessories, such as specialized filter holders (such as for extra wide angle lenses), adjustable lens hoods (Lee's is reasonably prices, around %65-75 last time I looked) and a variety of other things.
These use a filter holder and an adapter to fit the holder to different lenses, each sold separately. You'll need an adapter with 52mm filter thread, if memory serves. You simply need a P series holder for P series filters.
The filter holder rotates easily, plus you can adjust the rectangular Grad ND filter up and down or to any angle necessary so that the horizon line is where it needs to be. Just do this after focusing the lens and you'll be okay.
Do not get the screw-in ND Grad filters. They position the horizon line the same place all the time, making them a whole lot less useful. The standard ND filters are fine in screw-in type (and it doesn't matter if they rotate with the lens)... just not the Graduated ND.
Note: You also might look into applying adjustments in post-processing that can be even more effective and precise than ND Grad filters at the time of exposure. To be honest, I have a set, but hardly use my ND Grads any more. I just "double process" a RAW file in Lightroom & Photoshop - one exposure setting for the foreground, the other for the sky (or whatever) - then combine the "correct" parts of each via layers and masks in Photoshop. Since I rarely see a horizon that's a straight line, this gives more accurate results than any filter could, in most cases. Of course, if you don't already have the softwares, buying them would be considerably more expensive than a modest set of Cokin filters. Plus there's somewhat of a learning curve involved.