Oof.
https://www.mcmaster.com/product/87405K46-87405K6
$160 for a one inch diameter x one inch tall polyimide puck.
And i thought PEEK was expensive.
For comparison a 1" square x 12" long square PEEK bar is $140.
https://www.mcmaster.com/product/9089K144-9089K711
And an equivalent bar of 6061 aluminum is $12.
https://www.mcmaster.com/product/9008K14-9008K141
Other candidate high temp materials: Garolite (FR4-like glass-epoxy composite) is $30 for a 1x1 inch by 2 foot bar. About the same as 6061.
https://www.mcmaster.com/product/8557K612
But the glass reinforcement is hard on tooling and makes nasty airborne glass dust.
PTFE is $69 per foot.
https://www.mcmaster.com/product/8735K64
But ignoring all of the other issues with fluoropolymers, being super slippery is probably not a great attribute for a fixture that's supposed to hold things in place.