Help, vacuum source

I've just bought a Turbosmart dual port dump valve for my Octy vrs. After fitting i realised on full boost that it's not venting. I called turbosmart and they say that because it is a race dump valve, it needs it's own vacuum from the inlet manifold as the standard one is no strong enough.

Does anyone know were i can find a source of vacuum by the inlet manifold which i can plumb into?

Cheers

Si