The goniometer that I have came to me without a goniometer head, so I am now on a quest to acquire one. This brochure from Stoe has some interesting information.
I am told that goniometer heads with angular adjustments have fallen out of favor for use with modern X-ray diffraction machines. More compact heads with only X and Y translation are preferred. This implies that the heads which I would desire to find should be becoming readily available as surplus items.
Some makers are Supper, Huber, Hampton Research, and Stoe.
The Supper model 501 head uses standard 1/8 inch (0.125 inch) mounting pins. The distance from the base to the center of the arcs is 63.97 millimeters. The distance from the top of the pin holder to the center is 13.0 millimeters.
Hampton Research sells a new head with translation only for $620, which is a data point. I have seen used heads (with angular adjustments) on Ebay for $150 and $175.
Tom's Mineralogy Info / firstname.lastname@example.org