I think you are out of luck mate, they are not sold separately, so you will need to get either a broken one from a breakers or find a cheap one from ebay, I think there was a one on the bimmerboard forums for $100 shipped, I'll dig it out:

Here you go,

16:9 For Sale.

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.