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.
We noticed he had no friends. So we gave him this tiny ad:
A rocket mass heater heats your home with one tenth the wood of a conventional wood stove