What I do is get some freshly cut Pecan, Maple or Sycamore and spalt it myself. You can wrap it in plastic, sealing off the air, and it will grow as long as the conditions are good. Not to cold or it will go into dormancy, too hot and it dies. Elm spalts too, but the lines aren't as prevalent.
You can then turn it wet and bag it again, or slowly air dry it.
Alot of the spalted woods you would purchase are a crap shoot in that they will often get too far along in the decay process and are real punky.