If you go to McKenzie Taxidermy Supply and look under the tab Display Options or Habitat Supplies, you'll find all kinds of rocks, limbs, plants, mushrooms, trees, etc.
My next door neighbor is a full time taxidermist, and uses some of their habitat supplies. He also buys some of his supplies like vines, leaves, etc from Michael's. Generally he prefers to use his own driftwood, fence posts, limbs, or what have you, because he wants his mounts to be unique and not pieced together from a catalog. We'll take a few trips a year to gather old barn wood, fence posts, driftwood, etc.