Low poly modern building for urban backgrounds
The most common object that architectural visualization artists tries to collect on the web is furniture to compose their projects, but you should also try to find interesting buildings to place on the background your images. What do you use to compose projects? Sometimes a simple photo can solve the background, but when we don't have the right image the best solution is to create your own background. At some point I did a background composed mostly of old 3d models, because I didn't have anything else to place there.
Here is a model from BlendSwap created by an artist called boyleo that could be useful to compose backgrounds of architectural visualization projects. Its a modern look build that probably fits most urban environments, and has a low polygon count, which is great for rendering purposes.