"cusco rear strut brace e92 m3"
And the first link to pop up has a US Vendor. I'd post the exact link, but it's blocked.
And Vividracing has it too.
Grey Matter:2008 E92 M3:608/425. SG/PS SOLD!
Dark Matter:2015 F80 M3:495/505. AB/CB
? ? :2016 F82 M4 GTS --- Soon