There's an article in Trains.com about making trees using peppergrass. The author states that he gets it at florists and arts and crafts stores. I tried Michaels.com but the search came up empty.
Here's a link to the article.
http://www.trains.com/mrr/objects/pdf/mr_sc_5-05_toweringtimber_01.pdf
Good luck.
Steve
"...any man who may be asked in this century what he did to make his life worth while, I think can respond with a good deal of pride and satisfaction: "I served in the United States Navy." John F. Kennedy, 1963