C library provides functions such as write(),read(),open()...that we can use in our C program to make system calls. where can I find source code for these functions?
If you want source code for standard C library, look at the link @goldilocks provided.
If you want source code for linux system call, you must search through the linux source code. System call is defined in many locations of linux source. You can grep for string macro
grep -r -n SYSCALL_DEFINE /path/to/linux_source
Some of them you can see here.
There is good deep and detailed description about system call at kernel side in recent lwn article: http://lwn.net/Articles/604287/ .