Start by familiarizing yourself with
www.realoem.com. I highly recommend typing in the last 7 digits of your VIN on realoem, then create a shortcut on your desktop. This will make it much easier to search for part numbers and illustrations in the future. Once you're setup and have your model/year plugged in, select the category in which the part you're looking for falls under. In your case, it's "vehicle trim". Next, select the subcategory. In your case, it's "exterior trim". Scroll down and look at the illustrations. Click on the appropriate illustration. Note the items in the illustration are numbered. Find the item number next to the part you're searching for. Below the illustration, you'll see a parts list. Scroll down the parts list to the item number you've noted. Carefully determine if the description matches your year/model. If so, record the part number. In your case it's Part# 51132266094 MOULDING DOOR FRONT RIGHT not to be confused with part#51138236882 which is for a Non-Mtech moulding. Now that you've retrieved a corresponding part number, compare prices at getbmwparts.com, pelicanparts.com and ecstuning.com.
You're also going to need to order grommets, clips and a badge. Have fun looking up those part numbers