I've had good luck sourcing ceramic fiber board in that size from places like McMaster-Carr and CeramicFiberOnline. You might also check local refractory supply shops — sometimes they have offcuts or bulk deals. Just make sure it’s rated for the temps you need (2300°F or higher if it’s for a forge or kiln). Some sellers on eBay also carry those dimensions, but double-check the density specs.
