Need thin, non conductive, fire retardent sheets. Mica?

Hello,

I am looking for thin sheets that are non conductive and fire retardant for my battery box. I've heard and seen that Tesla uses mica sheets in their battery box. I looked up prices on ebay and amazon and the stuff is expensive!

Do you have any other recommendations? I need approximately 8 sheets that are roughly 36" x 20". So kind of a lot.

Thank you!

G10FR4 - comes in sheets as thin as 0.6 mm. It's fire retardant, and a good electrical insulator.

I just found this. MIght be the way to go.
https://www.electro-wind.com/rochling-glastic-gpo3-utr-1494-red-1-16x48x96.html