F.J. Gray sells heat absorbing glass in all sorts of sizes (www.fjgray.com). Because it usually has to withstand thermal shock from the light source, heat absorbing glass is most commonly tempered - ergo, it is cut to size and then tempered. The best stuff is also of optical grade with ground and polished surfaces. The heat absorbing qualities come from the tint of the glass and its thickness.
Cheers
Doug