It's good to get started by finding some decaying hardwoods that hold moisture in the wood. If there is moss and/or algae growing on it, they'll like it more. They don't want anything too wet, but just damp enough to the touch. Mulched, decaying leaves, pine needles, forest soils and wood with benign fungi like Green Stain, Dry Rot (or Wet Rot) or polyphores would be fine, since they like to eat them and it's not toxic.
Keeping a good population in the terrarium can be helpful as well, since they are good at keeping mold and fungus issues down, as well as cleaning up the droppings of the salamander.
Just as long as everything is gathered without having chemicals on it, you'll be fine. 5 gallons should be fine until you see crowding problems. Then you can upgrade to a bigger tote. A plastic tote bin is fine for a habitat, as long as there are a few small holes for some ventilation.