Create your own!
So what program do we want to execute? Heh! Let's spawn a shell. From the shell we can then issue any other commands we wish. But what if there is no such code in the program we are trying to exploit? How can we place an arbitrary instruction into its address space? The answer is to place the code we are trying to execute into the buffer we are overflowing, and then overwrite the return address so it points back into the buffer.