It COULD OF been one of many discussions on the subject!
The grammar is COULD HAVE been, of course. But when we SPEAK it sounds rather like we're saying COULD OF been. Unfortunately, many native speakers then SPELL it that way. Very wrong grammatically, but it does sound like it's right!
I COULD HAVE written it like COULD'VE, which is much better than COULD OF. Perhaps I should of ! Er, SHOULD'VE!
I think this is the article you're thinking of:https://www.italki.com/article/276/What-Does-Would-Of-Mean-Don't-Make-This-English-Grammar-Mistake!