Growing mushroom casing and spawn question

I am new to the whole growing mushroom deal I am interested in growing shittake mushrooms 2 questions:
1)Other than buying spores or mycelium from a supplier could I just buy some edible shittakies in the grocery store and use their spores or mycellium?
2) What is the best/easiest/safest casing to use for shittake Can i just simply use sawdust or is it an absolute must to use some other type of casing if my spawn substrate is rye grain? (is that a good substrate to use for shittake?)
    Thanks,     Matt

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