The panel is available I think but I think it is a massive job to replace it properly.
I just recently patched a small hole in my touring under the sound deadening in the battery holding lip. Instead of undertaking a painstaking welding job like I did on my iS, I used fibreglass this time. Time will tell how durable it is but it was a hell of a lot quicker, easier and less messy. Had to take the pedal box out to get at the job though.
It is still available, I have a brand new one that I bought with the intention of swapping out - thinking it was just a nuts and bolts job. It still sits in the garage.....
Don't think it was expensive though, maybe around £35.
1988 Tech1 325i Motorsport Convertible
2005 Porsche Cayman S