I have used two products. One is ciment fondu – a high temperture cement that is mixed with water and fired. The mould needs to be wet before applyng the ciment fondu.
The other product has been more durable. Its trade name is Sair Set. It seems to be a resin of some sort. Apply it to the two parts of the join and wipe off the excess immediately with water. My best results have come from leaving a supporting layer on the back – about 5mm thick.
Any ceramics supplier will also have some brand of high temperature cement for brick repairs on ceramic kilns that will work for moulds.