Hi, does anybody know some website to download models ready to use on Unity, because a need some props to use in my terrain like, rocks and trees.
have a look at these 54 website to download free unity assets
There is also: Where can I find art/model/assets for my game?.
Update: And in case you missed it, someone else has started a wiki question: Lists of sites to buy game assets.
http://unity3d.com/support/resources/assets/terrain-assets
If you're willing to pay for art there's also TurboSquid http://www.turbosquid.com/
That has a lot of open source art on it.
The best site I have found for this is Dexsoft Games. I use these models all of the time. Great quality, but sometimes the pricing is steep.
Example Packs and Bundles:
Best of luck!
You can set the filter for free if you please.