http://www.wagner-tuning.com/BMW-135...ines_p_42.html
Bought it from here. Got it shipped from Wagner's stateside retailer in Cali. Very well packaged. The Wagner has more surface area than the ER, is $400 cheaper (MSRP), doesn't require trimming, and has great customer service. One could also argue that it performs better.
My next step is to get the ER charge pipe with an HKS BOV. I understand that you like the ER better because it has a Z4 specific charge pipe, but that doesn't mean you also have to pay a lot more for an intercooler that does the same or worse than the Wagner.