Sounds like you probably feel you are on a "Snipe Hunt"!
Check local Nurseries for "decorative ground cover bark" and use the bark which has the smallest particle size (nuggets). Use only Bark product - not the hardwood stuff that is artificially colored brown, etc.
You can alternatively use Orchid Bark if you only need enough to make the 3-2-1 Mix for a few EarthTainers. (Can be expensive).
Raybo