The Gel Company
Go to Plates -> notched Alumina = $40
LGC UK
This stuff is pricey!
Bailey Ceramic Supply
Kiln equipment, no exactly what you are looking for.
Digikey MICA insulator
aaaah, this is the good one. Look at Keystone MICA insulator for transistor mounting, I believe fig 3 or 4 is what you are looking for.
RadioShack do sell this stuff, I saw it. Ask them for a MICA insulator for transistor (most regulators come in a transistor package). It is a little kit that even includes a screw, nut and plastic insulator to isolate the screw from the regulator.