Where can I find replacement covers for these vents from 1969?
That is called a Base Board diffuser.  They come with &amp; without damper. They come in standard lengths, 24" being one of them.  They come in white and brown.  They are available in various styles, heights and depths.  Amazon lists a number of them.  Also Supply House.com stocks several.  The HVAC wholesale houses will stock some and if you can find a salesman who is helpful I'm sure he could round one up for you.  If you go to a wholesale house consider taking the old one with you.  Happy Hunting. P.