i got the ones on ebay for $200.
BMW x5 E70 07 13 Arch Extension Fender Flare Aero Diffuser for 20 21 inch Wheel | eBay

fitment was perfect on the fronts.
the rears had some fitment issues but they send me new rears next day air once i showed them pictures.
i have yet to fit them as it got way too cold.
for $100 more you can get the OEM ones though.
i see you said you needed them because you are going to fit the larger wheels and want to abide by the laws and not have your wheels sticking out further than the flares.

you dont need the flares the offsets arent that aggressive on the larger OEM wheels anyway.
i put on 30MM spacers on the 19" before i put the flares on and still had half an inch before i would have been flush.