for manifolds, you'll generally have to go custom. or you can contact the other members of that forum who are using motorbike carbs and see if they are willing to sell you their prototypes. alternatively, you can google rowland manifolds, buy their dcoe weber manifold (was $180 including shipping, dirt cheap for a high quality manifold), and then just have a custom adapter plate made, which would be much cheaper.

on the other hand, im pretty sure bisimoto has a few motorbike carby manifolds.