The rules for Feral Orks can be found in the Warhammer 40000 Annual 2004.
I am using the Orc Command Sprue (from the Orc Boyz Regiment boxed set) to create some "cheap" Boar Boyz, but I am also using the remaining components to create some Feral Boyz.
I am also using the components from the WH40K Ork Boyz sprue.
The other nice thing about the spure it includes a standard. Having now got a box of Warhammer Orcs I will be making some more Feral Orks. Having undercoated the model, I have done the metal bitz and skin like my regular Ork Boyz.
I have added some more detail in terms of their uniforms and webbing.