It took me forever to find somebody who had insulating firebricks and would sell them to me (a non-corporate entity). They are usually under "refractory supplies" or somesuch in the phone directory rather than brick or hardware stores. I eventually scammed one place into selling me some loosies for $2.50 a piece. Later, I contacted a knife enthusiast at another refractory place who cut me a pretty good deal on 3 square feet of 2 inch DuraBoard for $30.
Actually, come to think of it, it's taking me longer to get the things together I need for my burner than it is for my forge.