I've bought the Sennheiser HD238 and didn't like how they sound . I wanted to "Mod" them and this is where your "Sennheiser HD238: Brief Review and Modding" came into help.
I'm not afraid to do the modding the only problem is that I don't know what are the materials you mentioned or where to find them : Soft Felt (Blue), Medium Felt(Yellow) and Thick Felt (Black).
Can you help me in any way ??
You can get felt from a hobbies/crafts stores. Art stores would also have them as well.