There are several manufacturers of 8 string basses (Hamer for one). Why don't you ask them what tailpiece they're using or if they'd supply one? Also try Luthiers Mercantile and Stew-Mac. Look into John Paul Jones, ex of Led Zeppelin, he plays one too.
As for string order, my gut tells me to arrange them so that the right hand finger strikes the octave one first, not because you want to hear it that way but because the path of the finger will then contact the larger string second with less chance of missing one of the strings. However, I don't think that's the norm, and flatpick styles might change that logic.