How can I get a copy of a judgement entered in a civil suit in DC Superior Court?

Check the court website to see if it was filed online.  If not, you (or someone on your behalf - if you have no one else, there are lawyers' services that do this all the time) will have to go to the clerk of the court to get a copy from the file containing all the papers relating to the litigation.  Since the judgment is so old, it may have been sent to the archives, but you should eventually be able to get a copy.