Some guys DO use the polyfoam rope, including Kithunter's current costume. It's inexpensive, and it comes in different diameters, though 1" is the most common for the Pred masks.
The rope comes in black, but some guys paint it with black or rubber plasti-dip spraypaints to give it a little different look. You can then create your own dred rings out of PVC pipe and a dremel tool with rotary saw to make designs with.
The reason many guys like the 2 part foam dreds made out of molds, is because it has a more natural "fluff" when you turn your head fast like in the movies....but it's a lot more $. The molded ones also taper down to the tip, where the foam rope you must sand down the ends to make the tip, or just cover them in tons of finger-armor rings like Kithunter did.
I mean like nylon rope? I used that for my first predator costume (which sucked) and in terms of movement they worked quite well. Just cut it in pieces, dip the ends in wax or even superglue to get the desired tip. Dreads on mine were just paper pieces rolled up (the camera quality was low so you couldn't tell) but I plan to custom-make some from cardboard or Sculpy clay.
You guys are correct of course, and in stereo at that.
I was SUPPOSED to have some larger diameter foam rope coming in the mail, but I guess I'm giving up on that.
So if I can't get Steve's box "O" dreds, I'll be having to find some of the larger foam rope somewhere else.
Hopefully I'll post an update in a week.