Bump.
My DT880s 600ohm seem to buzz quite a bit below 50hz in one or both drivers. When opening them up and looking at the worst one, the center of the driver does look to be pushed in a little (like you see tweeters in speakers in shops, but not quite as bad). They sound fine for most music, other than tracks with really deep sub bass. I do plenty of soldering at work and to me they look easy to replace. Can they only be bought by beyerdynamic at their prices?
I went through A LOT of Beyerdynamic headphones over the last few years and repaired many of them. The common problems that cause buzzing in the bass region are hair on the membrane (those can usually be removed), deformed coil (game over), unglued membrane (game over, at least for me, I haven't been able to properly glue it back in place), and deformed membrane, like in your case - if that is the cause of buzzing. It might not be, I've seen drivers with slightly deformed membranes that sounded fine so the reason could be something else. Also, I managed to repair less than half of the deformed membranes that were problematic (2 out of 3) by carefully applying negative pressure (with a straw) pushing, even pulling the membrane with a needle. You have to be extremely careful because it's very easy to do even more damage, especially that the membranes are covered with sticky coating. The 3 I could not repair - in one case I did exactly that, more damage. In one case I could not restore the proper shape. And I one I did, but the distortion persisted, probably the coil was deformed as well.
In short - you can try to repair it, but it's very risky and the chances of success are less than 50/50, even with a lot of practice.
And to answer your question: yes, you can only get new drivers from Beyerdynamic and their distributors at their prices, or sometimes second hand on auction sites. These drivers are actually made by Beyerdynamic, so it's not like you can buy a cheaper OEM version. I did experiment with using other drivers in Beyerdynamic headphones and actually had an OK result with drivers harvested from Philips headphones, but definitely not as good as original Beyerdynamic drivers. For DT880 you need drivers for this specific model (for DT770 and DT990 you can adapt drivers from other models, but not for DT880 - unless you manage to transplant the filter that's glued behind the magnet). And make sure you get the right impedance.