CS, there is no rule saying you have to use bark at all. I have plants growing in chopped up wine corks, tree fern, aliflor, gravel, moss and mounted on a piece of wood with no medium at all. First of all, what type of orchid do you have? Knowing that will help not only with the potting/ mounting but with light and water requirements. If you are set on using bark, buy the clean orchid bark that comes in a bag at HD or Lowes. The stuff you gather from the field may be contaminated with things you don't want your orchid setting in.
Jim
