Microsoft recommends searching through Bing to find clip art. At one point, a clip art database was offered directly through Microsoft, but that was eliminated in 2015.
Microsoft provides a detailed description on how it recommends searching for images via Bing for clip art. The most important thing is to make sure you are using images that are not copyrighted. Bing is recommended by Microsoft because it has a feature that allows searchers to pick an option that only shows images that are "Free to modify, share and use commercially," as well as a filter that allows you to search for clip art only.