The insulation in the photo looks like glass bonded mica. It is produced by several vendors throughout the world and is sold by many resellers. The vendor I am most familiar with is Crystex Composites (the old Mykroy Micalex). They have three grades, 400, 500 and 1100 that correspond to maximum use temperatures in degrees F. All are readily machinable using standard machine shop practices. They are significantly less costly than MACOR.
Crystex is located in Clifton, NJ. The telephone # is 973-779-8866. Resellers include Aremco Products, Cotronics, and McMaster Carr to name a few.
This is a fairly massive insulator. There may be a tendency for thermal stress failure over time. If this is a problem, you could substitute Grade A Lava. It will perform as an electrical and thermal insulator and have the stiffness necessary for your staking operation. It is also known as pyrophyllite. It is a naturally occuring aluminum silicate mined in large quantities worldwide. Any of the resellers listed above also supply this material.
If it were my project, I would start with the glass bonded mica. It is readily available, produced domestically, a good thermal and electrical insulator and the easier of the two materials to machine.