I've used a lot of the chennille for evergreen trees. Now I use Forest In A Flash for leafy trees they require assembly but I get 50 trees for $10. They can be sized to any scale. I Also use Woodland Scenics and I like their trees as well.
I buy my trees from Brooklyn Locomotive Works. 100 pines for $13.95, or 40 mixed trees for the same price. The quality is "good enough for wargaming" and you can't beat the price, service, or shipping.