Check out the assets of Mawi United. This company provides high-quality floral elements, trees, environments and other cool stuff.
Willi Hammes is the one man army behind this production studio. He worked in a number of high-profile studios, including Sony Pictures Imageworks, Blur Studio and some other big companies, His main field is 3d production and technical special effects problems. With Mawi United he builds amazing high-quality 3d assets and materials, which are specifically optimised for Unreal Engine 4. His groundbreaking stuff is not that expensive actually and you can purchase it easily through the official website.
Mostly the products present themselves various collections of plants, boulders, stones and other assets, which allow creating awesome environments. Everything is pretty affordable: prices range from 10 to 100 euros.