I am in process of building my HKT500 Heli, I would like to know if there can be any alternate servo to Turnigy MG930 (3.6kg/26g/0.14sec) ..... 

any 26g will do... futaba 9650, 9257, hitec 5245 or inolab ones.. .depends on what cost and your requirements are... cheap ones are also fitec ones and they are quite good (as per my personal experience)... www.rc-fans88.com

Thanks for the info Sujju, the link you have provided, is that supplier good ? Have you bought anything from them in past ?

Can Turnigy S3317M Servo (26g / 3.0kg / .14sec) be a good one for 500 size Heli ?? What does the "Kg" unit specify in the servo specs, if its 3.6 Kg or 3.0 Kg what difference it will make in the performance of the Heli ??

That is the torque (expressed in kg/cm)... and higher torque is preferred (when you have a choice). 

Unless you are doing extreme 3D, this difference of 0.6kg is negligible.  People are using the 9257, which is only 2kg of torque, without issues.