I would just like to look at the Metafont source code for the lower-case 'a' in the Computer Modern font. Where could I find that? Or even better, if that source could be the answer to this question.
Your question is not really well-defined, I'm afraid.
One answer is romanl.mf, right at the top of the file, but that file is run by being called by other files. You could start higher up the call stack with cmr10.mf, but even there you need to understand Knuth's macro definitions to be able to read what is happening. So basically it is one of those cases where to understand a small part you need to have at least a rough understanding of the entire system.