4-5 years ago I was able to get a small container of matching paint from the local BMW dealer that I got the bike from.
BUT, when I used it, I just wasn't able to really do a good job of matching up the work. There were several deep gouges in the plastic. Just recently I took my bike to a very small family owned body shop that was able to fix my repairs and a few scratches perfectly. They used their own paint and did a super job of matching it up. This was after several large body shops refused to even look at it because it was a motorcycle. Total cost to repair and paint the side case was around $150.00.