There have been extensive discussions on these forums about upholstery. The main suppliers who are listed in the Tech Manual pretty much source their materials from the same suppliers.
I am currently working direct with Hydes Leather (not a normal arrangement) to get a matching color and grade to the seat covers and door panels I ordered through German Auto Tops. After deciding to go with "Old Timer" Bright Red for my seats and not ordering the dash leather at the same time turned into numerous emails, phone calls and mailing of samples. The hide is coming from Germany but will first be sent to Poland to get skived before it gets shipped to me in the US. I expect to receive it soon.
In any case, if you haven't already done so, there are some decisions you will need to make before you order anything. The main decision is deciding what kind of leather you want. The standard offering from the main suppliers only compares in color to the leather originally used by Mercedes on the W113. Original leather was vegetable dyed. It had the color and finish on one side of the hide and was left natural on the back side. Today's more common offering is vat dyed where the color is through the leather. This can be an advantage since it is less prone to wear but also will not feel as supple as original. Then you have to request to have the material used to wrap dash panels to be skived otherwise it will not stretch and conform to the curves.
