Where do I file a complaint

For fifteen years, I have been purchasing our office computers through Dell.  My most recent purchase has been a nightmare and I would like to file a complaint with Dell but I cannot find anywhere to file it.  Where would I file a complaint?  If I can't file with Dell directly, I will go to the consumer advocacy groups instead.  But in the interest of fairness, I'd like to see what Dell has to say before I go that route.  And calling the 800 number does no good...I got a guy who took an hour of my time over an incorrect HDMI cord. So how do I complain to Dell directly??