Where can you get smell proof bags?

Your nose detects a smell when vapours in the air, given off by the thing which you can smell (pie, poo, whatever, as long as it has molecules which easily evaporate, hence why steel doesn't smell) react with exposed neurones in your nasal passage.

A smell-proof bag would be one that does not allow vapours sealed inside to escape, which is essentially any airtight bag. You can buy these in supermarkets easily.