I get it at local plant nurseries in bulk. Very cheap that way. @ZEROPILOT finds it at Home Depot in Florida, but I haven't seen it here in those kinds of stores. OSH used to sell very good stuff in 1.0 cu.ft. bags for about $12. You can get it at the pet stores, but its very expensive that way.
You'll have to look around to find a local source. People grow orchids all over the country. They are getting orchid bark somewhere.