I'm not sure if you can order a full replacement headband, but you can look into that option.
Also, Mark Lawton made a pair of Denons with a custom headband. He doesn't have a service for it, but if you contact him about it, he may do it for a fee. Also, looking at the design of the headband, it doesn't look like it would be too hard to make one yourself. Just remove the headband, remove the pleather from the internal structure, then take a fairly thin piece of glove leather and wrap it around some filler around the internal structure parts.
It's also possible that the pleather cover has just come loose. If you unscrew the metal bits on the side holding it in, you may be able to stretch it back out and rescrew it in place.