To make them lo poly you would need to add transmaps as leaf clusters , however, you would then have the problem of anim8or casting square shadows if you use any lights.
I have not come across any lo poly trees on turbosquid, the leaves are usually modelled and add the poly count to enormous amounts.
I have a Mr Smeagol tree somewhere that I could share, give you some idea, but it maybe better if you made your own trees, its not that hard!
Ditto: The tree generator does not work on WIN 7.
Was`nt there a plant generator or plantstudio that could make trees and things? Just thinking aloud here.
Some bits i found in my disk collection... They will be available on our Facebook Group.
https://www.facebook.com/groups/Anim8orWorld/