You can buy this diffuser on its own here in the UK, will require a custom exhaust though...but the diffuser is genuine at least.
A member on here has this diffuser, whit 135i with custom rear spoiler. He has a custom quad which looks great but the pics behine the diffuser show a tight fitment so you better use a good exhaust fabricator.
http://www.mstyle.co.uk/acatalog/info_B03AC047.html (ignore the product description, it's not ACS)