Grohe has a quite a few different shower hoses. The most common metallic hose is 28105000, and the the most common non-metalic hose is 28409000. These part numbers are for just the hoses.
I called Grohe and they do sell just the head holder. You can see it [linki]http://www.nationalbuildersupply.com/grohe-45650ip0-chrome~matte-chrome/p17918?src=search">here.
Grohe shows that the distance between the wall mounts is 24 3/8". Here is the spec sheet here