Take /usr/bin/free as example

rpm -qf `which free`
Search on Google for procps
Download it and untar. free.c is found in
/usr/src/redhat/SOURCES/procps-3.2.7

or

Search on koders.com for file:free.c

or

use command header literal string enclosed in printf and search on Google, e.g.

printf("total       used       free     shared    buffers     cached\n");