My Vampire crabs had crabs!

Well, I still have 4 baby crabs alive and seem to be growing. In a few months, when I can afford it or when my wife let's me buy it, I will be moving them all into a new 55 G aquarium. The 55 is ready I just need substrate. Does anyone know where I can find some cheap? and I also need springtails. Any ideas would be helpful.
May I know the substrate you used for your vampire crab tank?
 
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