I don't know how much you trust eBay for parts for your car, but I bought my BRAND new radiator from a selleer on eBay. I think the brand was BEHR.
Also, you might want to try the following sites:
www.bavauto.com,
www.bimmerparts.com,
www.oembimmerparts,
www.foreigncarpartsonline.com. I have especially bought parts from bimmerparts.com and they worked out for me.
MEH