Unfortunately you description of a Carver 4000 gas safety valve dose not define the component you are looking for. A picture would be helpful.
If its a gas valve in the heater, then I fear you may have a number of difficulties, the first, to find one as the appliance ceased production in 1999. and the second is because its a gas component, its removal and replacement needs to be carried out in accordance with the Gas Safety Regs and codes of practice to ensure its fitted correctly and tested for you and your families safety.
Try
https://arcsystems.biz/