Trees are one of those things that's hard to find at a good price or difficult to make and make it look good ("good" being a subjective term of course- many of us are perfectly content with green scrubbrush scraps glued onto a shaved pencil, and there is nothing wrong with that.)
But I prefer BIG trees. I want them to be at a more realistic scale. I want them to tower over my miniatures and remind the warriors that nature may be just the setting, but is still very much in control. (And when people can see the tops of the trees in your game from clear across the convention floor, they're gonna come see what's up.) Of course, trees can be too big and simply dominate the field. It's up to you to find the balance that works for your needs.
So let's start by going to our local hobby mega-store (Hobby Lobby, Michael's, Ben Franklin here in the states) or maybe your local floral shop and pick up a few choice plastic floral stems. There is a plethora of choices out there. The stems I have pictured here are from Hobby Lobby and are under the "Natural Brilliance" brand. These cost $4 each, but some stems can cost as low as $1 each (or cheaper if there is a sale.) You can get from one to three trees from each stem depending on what stems you choose and how full or large you want your trees to be when finished.
The great thing about these stems is that they come essentially "pre-flocked." So we have eliminated one fiddly and messy step already! Some stems will have large flowers, some come with buds, others are simply fern leaves. Use whatever suits your taste.