does anyone know where I can find a complete and reliable data of specific heat capacity for gases?..

The Handbook of Chemistry and Physics has tables spanning dozens of pages. Outdated editions are nearly affordable on eBay. Serves more purposes.

 

The CRC Handbook of Thermophysical and Thermochemical Data looks like what you need but is seriously expensive. Same for:

The Yaws Handbook of Thermodynamic Properties for Hydrocarbons and Chemicals.

 

If the your gases are manufactured by Merck, Sigma-Aldrich... they often give a heat capacity on their website. You might also browse the Web through the chemical name of the gas and "Msds"...

 

I've also read that some chemistry software for ab nihilo molecule computation calculates the heat capacity. Until compared with tables, I wouldn't rely on them.

 

"Complete" is impossible, and "reliable"... That's a human activity.