Best bet for headrests... eBay or wrecker's yard with national connection, like Potomac Auto.
Theses folks have been able to help with MB Tex replacement upholstery sections:http://www.gahhinc.com/
It's really not a bad DIY with their pieces (all pre-cut and ready to install). Call them for individual pieces; helpful and fair pricing.
Using the VIN plate of your car you can figure out the upholstery color code here:http://www.amcon-luxled.com/mbcar/index.html