I'm no expert, but here's what I've read/done myself:
You can check with landscapers in your area - they may be able to point you in the direction of where to purchase pure cypress mulch. You'll want to be careful - I bought cypress mulch at Lowes until I realized it could be mixed with different woods like pine (which is toxic). You'll want to make sure you check for pure.
I've read about other people using a soil/sand mixture, which you could pick up anywhere, or peat. The peat stays damp so it would require some maintenance making sure it doesn't mold, but it holds humidity well.
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