Is there a difference between orchid bark vs orchid potting mix bark? I've seen some at home depot unde r the miracle-gro brand it said on bag it was a 'coarse blend' bag. Is one toxic because it's a potting mix and regular bark isn't?
The potting mix is a blend of bark and soil. It would be ok to use if you don't mind the soil. Sometimes it gets pretty messy. I would be willing to use that mixture with box turtles, as I use potting soil as their bedding so they can bury themselves.
You won't find what you need at HD or Lowes. I've seen it at OSH, but its in small bags and somewhat expensive.
You'll need to go to a nursery or garden center for bulk fine grade orchid bark.
I would not use anything with soil in it for tortoise substrate because store bought soil is made of composted yard waste and there is no way to know what it is made of. Could be totally fine or it could be toxic plants sprayed with toxic insecticides. No way to know.