CN Entertainment publishes all kinds of 3D models in the Unity Asset Store. These include buildings as well as modular sets that allow game developers to easily create individual worlds.
Modern house with pool
Modern House (only exterior) with Garden and Pool
Low Poly City: Package
Low Poly City package with 70 prefabs and 17 materials. A lot of customization is possible and the package is very modular. It is very easy to use to quickly build a large customized city.