Sometimes they're metal, sometimes plastic. I assume you want plastic, likely white.
http://www.zorotools.com/g/00056803/k-G2548165?utm_source=google_shopping&utm_medium=cpc&utm_campaign=Google_Shopping_Feed&kpid=G2548165&kw={keyword}&gclid=CLmAtd2j370CFY17fgodoz8A8g
I googled "plastic hole covers" but only looked at this one image and clicked on it.