Hi Jerry,
Mixed species forests are difficult to construct in a way that looks convincing.
I am not exactly sure if your problem is in locating species or sources or both.
Joanie's right that Evergreen Gardenworks is a good choice as a source. Some area nurseries carry trees in 4-6" liners for bonsai. They come from
Miniature Plant Kingdom. Or you can head out to the source in Sebastapol (near Petaluma?) and visit a couple of nurseries out that way like Lone Pine Gardens.