I got mine at Home Depot, paint department (you have to know to go t the paint department!). It is hygroscopic, meaning it will suck water out of the air and ruin it, so I have mine sealed in big ziplock bags with Silica Gel.
Zip lock (low density polyethylene) bags are good for short term storage at room temp or long term in a fridge/freezer (have you noticed though that many foods appear dried up after very long term freezer storage). They are not vapor proof and will overtime in a humid environment allow water vapor as well as oxygen (a spoiler gas) to pass through. Silica gel is a great dessicant but the hygroscopic TSP will most likely absorb more water vapor before the silica gel will. Glass or high density polyethylene containers are the best.
Interestingly, cellophane is much better at protecting from oxygen then low density polythylene. I would imagine it would be better for water vapor too.