I just checked Ebay UK since I live on the other side of the pond and found this:
BMW Meyle Power Steering Fluid Reservoir Bottle Tank with Cap 3146320000 | eBay , is Meyle known to any of you? I'm not going for the cheapest but I'm also not getting too many options (on ebay atleast).

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BMW X5 E53, 4.8iS - 2005
Professional Stereo/Nav with DSP (modified BM54 module) - Converted to Eonon GA5166X with Reverse cam
4-Corner Air Suspension - Converted to BC Coilovers