Plenty on eBay, such this BRAND NEW & VIRGIN TRANSPONDER KEY BMW 3, 5, 7, X5, Z4 ,REMOTE KEY HEAD FOB | eBay
I forgot the link to a guy in Sydney who's also selling it on eBay.
Rob, there's no such thing as "OEM key". BMW holds exclusive right to their key fob (that's why it's $600+), but it couldn't stop the Chinese to copy the technology.