Try NIST chemistry webbook

http://webbook.nist.gov/chemistry/

Thank you so much Dmitri.

But there are compounds for which the thermodynamic data is missing like carbon and NbC.

Heat capacity coefficients for solid carbon (graphite or diamond) and NbC can be retrieved from the above signalled reference, i.e.: O. Kubaschewski, C.B. Alcock, P.J. Spencer, "Materials Thermochemistry", 6th ed., Pergamon Press, 1993; cf. pp. 265 and 297, respectively.

Mr. Carlos, the mentioned reference is not available online even on googlebooks. and the values of coefficients differ from source to source, one should take care in adopting the values from single source.

Do you have this reference?

Try to find from 

http://www.chem.msu.su/rus/handbook/ivtan/welcome.html

but it is in russian

The previous editions of the Kubaschewski reference use to be called Metallurgical thermochemistry.  The fifth edition has thermo data for the compounds that you are looking for.  You should be able to find the 5th edition at a good university library.  Also note that the free energy values (G) in the 5th edition are tabulated as chemical potentials and not free energies of formation.  This sometimes trips people up. 

If you have access to HSC Chemistry software the same data is there as well.  They also tabulate free energies as chemical potential.  If you are use to using free energies of formation the program can easily convert the data for you.

Thank you Mr. Emerson