Far be it from me to point out the obvious---but if you aren't entitled to a reason as to WHY you were fired ( not a right-to-work state) then I'm gonna go out on a limb and guess you aren't entitled to a reason in WRITING either.

No employer with an ounce of self preservation in their bones would put into WRITING anything, especially since they aren't compelled to by law. Nothing like handing a disgruntled employee something a skilled attorney can scrutinize, analyze and twist around to use as grounds for a civil lawsuit.

Follow the logic--if you can be fired WITHOUT NOTICE for ANY REASON, asking for a REASON in writing seems like an exercise in futility at best, and contrary to popular misadvice, you AREN'T entitled to anything in writing.