Identifying Proper Parts (HID Ballast and Bulb)

Hi all. Long-time lurker, first-time poster.

I know very little about car parts. I need to replace my HID ballast, and want to order one on Amazon (rather than paying the $400 the dealership is trying to charge me). I figured I would also replace the bulb at the same time, while the headlight assembly is disassembled.

However, there seem to be multiple parts advertised as fitting within a given year. For example, bulbs are offered in D4, D5, etc. (For reference, I have a 2008.)

How can I determine what parts are necessary/will fit in my car? Is there a repository? Should I be checking something with my VIN?

I'm sure this question has been asked before, but I couldn't find an answer by searching the forums. Sorry if this is a repeat.