Can't say for certain, but I'm reasonably sure there was an add for a DTM Mercedes in one of the British classic car magazines in a recent issue. I don't have the issue, so sorry, I can't check, but it would be a good place to check. Some of the cars are being used over the course of a few seasons, but they're are quite a few of them, so I am sure that they can be obtained.
As a matter of fact, a quick google search brought this one up...
http://www.carandclassic.co.uk/car/C07649?pt=pf