I recently took advantage of a deal on The Miniatures Page for some Flames of War terrain that was 33% off retail. I needed to step up my game a bit for my Vietnam project, knowing that while I had some trees and scrub from my modern desert projects I could use that is wasn't enough to make a full and nice looking table. So, I snagged two boxes of jungle bushes and two boxes of island palms. The Flames of War terrain is nearly ready to use right out of the box...for the bushes I simply needed to pop them off of a sprue and push them onto a peg on the finished base piece, but for the trees I needed to chopped down the stem so that they would fit into the pre-drilled holes on their bases.
I am also working on a box of Pegasus banana trees that I have owned for probably ten years. These are a bit more labor intensive, needing the pieces cut from the sprues and then having the leaf pieces mounted on the trunk. These trees are vastly smaller than the palm trees from Flames of War, but I still think they add a bit more "ummphf" to the visual display.
Lastly, some bamboo pieces I picked up on an eBay shop based in China. These were from a recommendation by Garryowen over on the Fields of Fire Vietnam gaming forum. Very cheap (just a few bucks), I am taking the various lengths and creating bamboo groupings.