bin sbin etc are where the system keeps the programs once they have been compiled.
If you have a major distribution with its own packages managment system then you may not even have the source code to the program installed. [I don't know how deb packages work]. If the source is on the computer, its probably under /usr/src,
ps is part of 'procps', and the source packages are available at http://procps.sourceforge.net/