Interesting Brent, I will check out some camera shops for these bargain filters. Might even just be worth doing that to see if I can get the filters and pop them out of their threaded barrels to use the barrels for my purpose.
However, I have just found this technique which can be used as well for that "silky water" look without the need for any filters at all. Basically take approx 15-20 normal exposures during the day and then stack them in photoshop. The result is pretty much the same as using an ND Filter. Link to the video describing this technique:
http://www.youtube.com/watch?v=09njDK4TsEw