Materials

Material resources

List

https://www.eevblog.com/forum/manufacture/tough-sheet-dielectric-material/ - Discussing paper sized dielectric materials, I read through some of this. There are many options discussed.
https://web.mae.ufl.edu/designlab/Lab%20Assignments/EML2322L-Tolerances.pdf - Typical Tolerances of machining processes. Looking at this short doc, you can see that anything that requires the highest tolerance will be ground down by some sort of surface finishing. That was what was used for polishing the ill fated hubble mirrors according to "The Perfectionists: How Precision Engineers Created the Modern World" by Simon Winchester, which in that book was rated as the highest level of machining tolerance. Rated higher than semiconductors, though that is debatable.

https://ferroceramic.com/ This website has a list of 15 different ceramic materials that they machine. There is a writeup on each material and it is an interesting intro to industrial ceramics that are actively used. Silicon is mentioned. Some others: Macor, Mullite. Note that these are smaller parts (20"x20"x8").

https://www.approvedsheetmetal.com/sheet-metal-fabrication-images These folks look like a fair sheet metal fabricator. Available for parts in single qty.
https://basiccopper.com/thicknessguide.html - Metal (copper) Foil Thickness guide. Remember that what applies to copper may not apply to other metal foils.