Here is a list of trees I think are the best from the 3D warehouse. If you have already downloaded these, please forgive me. http://sketchupland.posterous.com/best-3d-trees-from-3d-warehouse
Depending on what you are doing, 2D trees/shrubs are good alternatives. Some 2D picture “face me” plants can look just as good or better than a 3D low-poly plant in 3rd party rendering programs. They also keep the model small and easier to manipulate.
There are some free models on the XFrog site, but they are HUGE and need to be edited for SketchUp use. (Download the .3ds files and use Fixtreed ruby on them.)
Form Fonts has an “exchange” part of their website, but I don’t know if it restricts the use of the components or if there are plants of any value. I’ve heard good things about their plants that you buy (low-poly but good quality).