Drew – Working in a CA firm we struggle with this as well. We have had to create most of our plant cutouts from our own photos. Here are two resources we have been able to pull some plants from. Not sure how many CA natives they have but worth a shot:
http://www.sketchuptexture.com/p/cut-out-png-resources.html
http://www.gobotree.com/ [search the cutout, plant option]
Hope this helps. If I think of anymore I will post them.