it will say it on the bag--fir bark, or bark nuggets, or "orchid mix" which i frankly think doesn't have enough bark in it; when i do a mix, i throw in a spoonful of that just for the clay and charcoal, but i go majority bark.
this sort of thing should be with all the bagged potting soils. sometimes it's also labelled as a decorative top cover. for instance:
http://www.schultz.com/ProductCatego...s/PottingBark/ http://www.schultz.com/ProductCatego...nts/OrchidMix/
happy hunting!
(oh, and do NOT buy something that's actually labelled as "mulch"--that stuff is often from poor quality wood, like leftover telephone poles and things, which have been treated with creosote or other chemicals. that stuff's ok as a ground cover outside, but you wouldn't want to pot in it or have it in your house. so don't be seduced by the really cheap fifty pound bag of mulch.
)
oh--and you may want to go to a nursery or someplace that deals with lots of orchids and look how their cyms are potted. (local botanical garden maybe?) also, keep an eye out in your area for nurseries that are holding orchid festivals--they'll often have repotting clinics or offer free repotting and you can see how some of this is done.)