I think the best is the fine grade sold as "Orchid Bark". It is evenly graded and makes a great substrate. There is also the exact same material sold as "Micro Bark" for landscaping. It is not as uniform a grading and has more of a mix of fine to medium bark. It is 1/2 the price so if you have a large enclosure, that works well. I actually use that one as I have so many large enclosures. I just bought 4 bags on Monday from Green Acres in Elk Grove.What size Orchard bark do you use in your enclosure? I am headed to Green Acres this afternoon, there isn't an issue with the torts trying to eat it?
To help decide how much - I use about 1 1/2 cu ft for one 8x3 enclosure.
Here's the nicest one that is about $11 for a 1 cu ft bag. Normally kept inside by the houseplant section. Be sure to check the bottom right of the bag where the "fine" is checked. Medium looks identical except the "medium" on lower left is checked.
Here's the less expensive one that is not as well graded, but still works great. $10 for 2 cu ft. Kept outside with the top soil, bark products.