I don‘t have time to wait for a mail order, but I can‘t find a store that sells these items.
Where are you located? A ceramic supplier will have a ceramic fire brick. But without your location, I can't suggest one near you. You can try a local craft store or art store, but this may be more of a specialty item. I use Rio Grande.
Search nearby your address for Refractory Suppliers - who do fire brick, frax blanket, etc. - although insulating fire brick (the light stuff) is often sold only in box quantities - ask if they have cutoffs from creating cut down brick shapes. An alternative would be Fireplace and BBQ places (we have them in my town) that have the materials for construction. Vermiculite is fire proof and used more often for annealing beads and is available from gardening supply places in varying quantities. It is very light and fluffy (perlite is heavier) and in the garden holds moisture in a soil mix.
