You can buy springtails from any number of online stores for the hobby. Josh's Frogs and NeHerp are a couple examples.

Eggcarton --- I assume you are referring to eggcrate which is only used in tanks where you are creating a false bottom (a False Bottom is where you raise the bottom of the tank up to create a void for a water reservoir. ) If you are not creating a water feature than you do not need this. Eggcrate is actually a light defuser panel for drop ceilings and can be found and big box hardware stores.

Sounds like this will be your first natural Vivarium (Vivarium is a terrarium designed for living animals to live in). You might want to take a few minutes and read through the Vivarium Construction 101 thread to see the common steps and methods used in building a vivarium.

Any questions please feel free to ask and when you start building your tank create a thread in the Vivarium section and share pictures as you go and ask all the questions you need

Oh and welcome to the FF !!!!