If you're looking to get just the low beam themselves, any D2S bulb will be fine. For the most part I've always seen them being 35watts. I had the stock 4300k and got some 6000k off of eBay for like 10 or 15 bucks, I forgot.
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745Li E66 Sterling Grey Metallic
/ 35% Tint (windows) / 20% Tint (tails) / ACS Roof Spoiler / Interior & Exterior LED's / MTEC V3's / 6000K D2S /
DDM Tuning HID 6000K Fogs / BIMMERFEST license plate cover