You may not find many C source-code files on your system if you have just followed a standard install. Some may be available from the Fedora package manager (e.g., if I want the source code for my current kernel, I can download the linux-source-2.6.24 using Ubuntu's package manager). For sources for other parts of the system, you may have to go to the site of the project that developed it. For instance,
The GNU core utilities: http://directory.fsf.org/project/coreutils/
Gnome projects: http://projects.gnome.org/
If you haven't already found it, the file
command is useful for identifying particular types of file. You could use it with grep
filtering the output to identify all C files in a particular directory:
rob:src$ file * | grep 'C program text'
base64.c: ASCII C program text
basename.c: ASCII C program text
c99-to-c89.diff: ASCII C program text
The advantage of this (as the c99-to-c89.diff entry shows) is that it doesn't depend on the filename suffix.