There is a sheet metal intake for sale on ebay. Here is the description:
Looks like they are located out in Tampa, FL.Fabricated Aluminum Intake FOR BMW N54 ENGINE 335i 535i Intake manifold with fuel rails. These intakes have been updated with
3mm sheet aluminum and will not bend or flex under vacuum or boost. Fuel rail included.
Link: N54 BMW TIG WELDED SHEETMETAL FABRICATED INTAKE MANIFOLD 335i 535i twin turbo | eBay
Interestingly, there is also an identical looking manifold available on chinese wholesale site Alibaba: Bk-3413 N54 Billet Intake Manifold With Fuel Rail Kit Set - Buy N54 Intake Product on Alibaba.com
I've also contacted the Alibaba seller to get a quote on a single unit.