Hey, welcome to the CG, wisp.
1. There are some good brush sets both here and on Deviant Art that have decent trees/forests in them, especially StarRaven's set(s). If you have a graphics program (heck, even just Paint), it's fairly easy to use them. Ultimately though, the best advice I can give is to learn to draw your own. It's great using other folks' brushes or clipart (as long as you have permission), but eventually you want a style all your own. Of course, the level of artistic ability differs from person to person, but I really think just about anyone can learn to draw a decent tree or mountain. It just takes patience and perseverance.
2. Someone here at the Guild invented a program called The Tree Thing, and I cannot for the life of me remember who it is, but if you do a search for 'tree thing', you should find it. It might be just what you're looking for.
Hope that helps.