Quinsis in Belgium is good at that. They have much experience in higher quality sturdy risers, by using clay as part of the binder. Their highest specification product is 1400º C, one builder in the Netherlands is using those in board form inside the firebox of batchrockets.
http://www.quinsis.com/
regards, Peter
The truth is rarely pure and never simple - Oscar Wilde
building a permaculture community with hands on gardening and natural building