Speed tree and plant factory both sell trees. They have some great stuff. And of course x frog. All the good stuff costs though, and I don't know if you have to have speed tree and TPF to use their stuff or not. But I know you have to pay.
I think it costs lees per tree than buying from places like turbosquid, for example.
I don't think I have ever seen a tree of this kind for free anyplace at least not a a truly good quality. Silva has very good prices, maybe the best, but I am sure walli does not have a tree like this kind on his site, unfortunately.
To be sure, if you want a really high quality object of his kind, you will have to buy it or make it your self.
On the bright side, you can do both. And the idea in your image would be worth the cost or effort, imo.
