I was in the same predicament not too long ago, so what I did was I bought a box of Black Orcs from the Fantasy line. Unfortunately, you'd have to buy 7 boxes of warbikers to be able to get that many, and not too many people buy that many boxes and not make the nobz..... You could also use Nob torsos from the Boyz box as well, but those have that funky raised right arm that makes putting a weapon on it (or holding a handle bar) rather difficult....
Oh, and I also forgot you could always buy it from the Warstore Battle Bits section. He has it where you can buy the nob....http://www.thewarstore.com/product33662.html
Kinda expensive (you'd spend almost 80 bucks buying 7), but might be a better choice if you don't want to buy the Black Orcs and have to extensively hack apart the torsos like I needed to.
A tiny portion of humanity turned its back on mankind in the waning days of the last Golden Age. Over one and a half centuries later, the PHR has emerged from the shadows as an unrecognisable civilisation, its people irrevocably changed. They are no longer simple human beings, they are post-humans - cyborgs.