I heard about this joint a few months back but never got to go there, maybe others already know about it but here goes anyway. Its called Lagrange materials in Lagrange. They had regular and split firebricks. Both were $1.25 each. Its just east of downtown Lagrange, just north of 47th street.
They won't cut them for you though so I stopped aT a granite place by my house and he cut them for me for free. 2 I got cut in half and another I got cut into thirds. Both fit perfect in my small.