Find old stock certificate that belonged to deceased father

My father passed away a few months ago. In going through his belongings, we found a stock certificate from 1996 issued by a credit union that no longer exists. It looks like the stock is a company stock the he worked for. I contacted the company from which he retired about 11-12 years ago. Fortunately, I spoke to a woman in HR who remembers my father and has been willing to help. Their investments are now issued by a different bank (his stock is not there). I was trying to find out if anyone in the company remembers when the credit union shut down where the stocks were transferred to. Whoever would know that info from the late 1990's no longer works there. I am trying to find another way to research it to see if my father has some stocks or fund floating around somewhere that we are not aware of.