I’m interested in this too as I often design landscapes and want to populate them with realistic renditions of specific plants. I don’t know of a good resource.
Ready made components are usually either too basic, too generic, or too detailed. By “too detailed” I mean that they have a high poly count that results in a bloated file. The problem with plants is that they aren’t made up of squares and lines but need double curvature and multiple facets to get anywhere near realistic. I think that’s the basic problem.
One way around this would be to get a picture of the trees and shrubs you want to use, scale them appropriately, and then make them into Follow Me components. That would significantly reduce the filesize over 3D components.