Has a decent number of tree billboards, mostly of Australian trees.
Drawbacks: the alpha maps are poor, so they don't blend well, and
they are oddly supplied in formats like OBJ, DAE, 3DS and SKP instead of
as simple textures.
Blender Textures: Vegetation has around 16 plant billboard textures,
although most look a little strange and artificial
Got3d Free Textures - Free Tree Textures has around 180 side-view images
of plants, which could be a starting point for making billboards, although
there is no information about what species they are
USDA PLANTS Database has a lot of
plants, but just a little information about them; plants common in the USA
often have detailed characteristics described such as Height and Leaf
Retention
Gernot
Katzer's Spice Pages has excellent information on 117 "spice" plants
(including coconut and orange) with an index of 4000 common names in about
40 different languages
"Pro-Viz Plants Textures Xtreme Bundle" (£64) and "Pro-Viz Trees
Textures Xtreme Bundle" (£36) from
3drender.co.uk contain hundreds of plants and trees at a modest license
cost
3D Nature's
Realtime Foliage Billboard Library CD ($1500) contains 1100 images, of
which around 950 are plants. This is the same library that is included with
3D Nature's WCS/VNS products.
ImageCELs has several CDs of
trees and shrubs, $1000 for the collection
Digital Nursery sells
individual images. As of 2006, there are around 16 deciduous trees, 24
palms, and a few shrubs. Images are $3.50 each.
CacheForce has "many sorts of
plants (100 models, trees at hi and med res)"