From Alloy Avenue. Tips on where to get and how much to pay for foundry materials.
If youre looking for refractory, you can usually find it at boiler installation and repair comapnies - try the yellow pages. Most companies will stock refractory and many will sell it to you by the bag. Usually about a buck a pund is a reasonable rate. Firebricks can be found at pottery suppliers (used in Pottery Kilns - youll want the insulating kind which are doft and full of air pockets like a sponge - versus the dense kind which are immesurably stronger, but transmit a lot of heat). You can also find the brick at a fireplace and masonry supply. Another very successful strategy thats been posted recently for insulating your furnace is using LECA (Lightweight Expanded Clay Aggregate). You can find this at a nursery/aggricultural center or online from Hydroponics suppliers.
In the same thread, Nudge says that the lid should NOT have rebar in it, as they just heat/contract and end up cracking the lid. Better to have a metal retaining ring to hold the castable in the lid rather than rebar.