Update
The ones in the link from "alexcoppo" are also found in the link to autodesk.
I suggest you use alexcoppo's link, since they have the Vue readable formats for them.

But there are many more available at the autodesk site, 100 different species (with 3 or more versions for each.) as opposed to the Xfrog one with 30 species (130 plants). But you'll need to convert them from MAX, MA or XFR file formats.