Cattail seeds or plants would be a waste of money....
Just find some, harvest the seeds and sprout, then plant.
Or better yet, dig up one of the root masses, and plant that.
The problem of course, is that cattails may not be able to handle the
stream if it has much current at all.
A small dam, might be enough to slow the current to allow the growth,
and since, residence time is the imporant factor in cleaning up the
stream in question this would double the effectiveness of the plants.
.. something like big river bulrush, or Burr Reed,
might hold up to current better, these are rarer plants and you should
order the root stock of these...
green ash, and willows will also help in reducing the nutrients before
they reach the stream
willows can of course be planted right into the stream to hold the
I would advise caution planting cattails where they don't currently
adding a small riffle area with rocks would oxygenate the water and
help clear it up as well.
How big of area are you talking about?