If you need high purity alumina insulation material (because your process is in hydrogen atmosphere) than contact Zircar Ceramics http://www.zircar.com/index2.htm
They have a variety of HA boards, from ZAL-15AA (15pcf), AL30-AA (30pcf) to ZAL-45 (45pcf).
Unifrax at http://unifrax.com/ has also a...