Quixel Speargrass this for example is designated as #3DPlant in the lower rt corner on the main page, this should be an actual mesh. Also if you dont see what you need in megascans the free assets that can be had w/ Unreal should have some 3d grass.
https://quixel.com/megascans/library?returnTo=%2Fassets%2Fofivdfoh2&search=grass&search=3dplant try this and search for grass and 3dplant and you will see the 3d meshes of grass
Hope this helps,