I like the blocks of coconut which you just add water to I find them great imagine you mean that. I buy them online .
I mention that just incase you rather not buy at petstore . Either way I would use moss sold at garden stores they have large bags cheap pure moss no branches no junk but it has bugs thus I would boil it first than dry it or soil with NOTHING added no vermicul or anything added.
The mulsch is lethal as full of harmfull oils not to mention splinters sharp as glass