CMC is carbyl methyl cellulose. It is a binder (glue) and it helps keep solids suspended in liquid. You can buy it at a pottery supplier as it is often used in glaze mixtures and known as a glaze binder. It normally is supplied in powder form.
To make use of it you need to dissolve a teaspoon or two in a half pint of water and let it stand. By the next day it will be a thick mass. It can be further diluted, but it is often best to make it to the consistency you will need for the current purpose.
Klyr Fire is said to be a 3% Mixture of CMC and distilled water.