i got them here to replace the xenon bulbs on my 6 series and they were perfect , for a fraction of the cost , as said replace them in pairs

http://www.powerbulbs.com/eu/product/xenarc-d2s-single

http://www.powerbulbs.com/eu/store/category/xenon-hid-bulbs-d2s-d2r-d1s-d1r/fitting/d2s-85122