How do i make a rubber compound that could help me to make mouldings?

My actual need is to know some sort of moulding material that is flexible like rubber, from which i need to get a mould of some carving which have curves and groovs. This moulding material should be easy to find and readily available anywhere and cheap. What I have in mind is some material like fiberglass which is used for moulding but should be flexible like rubber. May be if there is some possibility to use some rubber compound that could harden once poured on to an object to obtain a mould of the object. ( Simple need is what material could be used for this purpous or How I could produce this myself )

Google "RTV Rubber" and you should be able to find a local supplier with a little searching. Or, check for both supplies and really good information.
You beat me to it - smooth-on's got tonnes of casting silicones/rubbers.  They vary in strength, elasticity, temperatures, viscosity, etc...

For hobby scale, or larger - they are probably the best/cheapest source.
Yup. You can sometimes find good quality silicones for cheaper on eBay, but that's hit-and-miss. And the huge variety of formulae combined with the loads of info on the site makes smooth-on the best choice for an internet supplier, IMO.