Besides the Dollar stores, you can usually find fake plants at some of the arts and crafts stores like Michaels. Where I live, there is an entire store devoted to fake plants called House of Silk Flowers. I don't know if it is a chain or just local.
The best place that I have seen for getting bark and branches is reptile shows. There are usually one or two vendors at the shows I attend that bring in large selections from small pieces to large pieces for big custom enclosures.