You can get the full ND filters, which will slow things down depending on which one you get, 1 stop, 2 stop, 3 stops, or even more... like 9 stops from using the hoya NDX400
OR there's a B+W one as well.
I just bought a 1 stop grad nd and 2 stop grad nd, they are plastic filters by cokin for which I need to use the filter holder and adaptor ring. I bought them online as Cathay Photo had no stock. I bought my adaptor rings seperately from another source, and they havent arrived yet, so I have to hold the filters infront of the lens for now
There are also the screw-on ND filters available. and some can be rotated