Restaurant supply stores, online, etc. They're called "BiBs" or "Bags in Boxes". Usually $80-100 each. Obviously meant for restaurant use, I don't recommend buying it yourself you couldn't make that much soda before it goes bad. But one of those bad boys will make ~120 liters of soda.

SodaStream has begun selling Pepsi syrups for home use. If they weren't an Israeli company I'd buy them.