A number of Macor distributors are online, though, annoyingly, Corning's website does not list the links. I compiled what I could - hope that it turns out to be useful to someone else out there. Note that Leed Plastics has been purchased by PlastiFab.
Circle Fabrication, Inc. (website down)
P.O. Box 153
Holcomb, IL 61043-0153
Nancy Nichols (email@example.com)
Technical Products Inc.
P.O. Box 189
Hubertus, WI 53033
Gregg Gumness (firstname.lastname@example.org)
Astro Met, Inc.
9974 Springfield Pike
Cincinnati, OH 45215
Frank Gorman (email@example.com)
Macor is a machineable glass ceramic produced by Corning. It has a DC volume resistivity at 25° C exceeding 1016 ohm-cm. That puts it in the same category as sapphire in terms of resistivity. It can be machined with ordinary metal tools and requires no firing after machining. There are only eight distributors in North America. Astro Met in Ohio has a price list posted online. It ain't cheap. 1/16" is probably the right thickness for my application. A 3"x3" block cost $51. The closest one to me is San Jose Delta.