Well, I could verify that you still do not have the item "head texture" in your inventory, you can buy it in this section of the market:
Head Textures
Once you have this item, you can buy the texture (only the image) in this section of our forum where users sell custom head textures:
Art/Textures
Note: The simple head texture is in 128x128 resolution, so if you buy a texture in the arts section, make sure that the size of the image corresponds to your item.
Also you can purchase textures in 256x256 or 512x512 resolutions, but it is not the same thing.