I know three: @Trickyy90 @Coolpants and @Buugi
First two built for me already with excellent effect so I can vouch for their skill and imagination.
Buugi posted a lot of his work and I was always impressed.
I cannot vouch for their readiness to take on your project though - they might respond with yes/no when they see this.
And then there was this excellent contest resulting in many players building various trees to create unique forest:
Maybe one of the contestants would be interested? It’s a great source of screenshots of tree builders work, so maybe you will like one of them enough to make contact and ask if they would build for you?