Needing help on substrate!!

[Tahlia]

Hi all, I’m wanting to make my 3 small dumpy’s a full bio active Vivarium (isopods, springtails, plants etc) for a 60x45x45cm reptile one rainforest viv but I’m having a hell of a time finding the substrates to make it I.e. tree fern fiber can’t be found here that I’ve found for more than a 1 pound bag for $35 + $10 shipping and having a larger sized Vivarium it wouldn’t be enough/I don’t want to spend a ridiculous amount on it.
What alternatives do I have to the ABG mix? (Atlanta botanical garden mix)