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
He was giving me directions and I was powerless to resist. I cannot resist this tiny ad: