Are you talking about an exothermic process like CADWELD or THERMOWELD?
If so the molds are made from a fairly dense graphite block and are usually machined except in the case of the smaller ones that my be compacted. I don't know which process is used to get the block.
We purchased the molds from the manufacturer of the welding powders and usually got 50-100 shots out of each mold.
If you have a different process in mind or need a special you might give these people a call.
I imagine there are quite few of the graphite producers that handle you requirements.