The Soda Stream instructions specifically mention that you should only fizz clean cold water. I assume this is because the fizzer can become fouled/dirty and I don't see a way to remove it for cleaning. In fact, there are no cleaning instructions either.
The colder the water the more co2 the water absorbs, so if we finish a bottle before dinner we put cold water in the bottle and stick it in the fridge overnight.
You can also go to restaurant supply stores. Kind of like costco for restaurants. They have the syrups... however you usually need to present some form of business ID like a state tax form before you can buy there. At least in VA its like that.
Gordon Food Services sells to everyone. I think there's a discount for wholesale, but they also sell retail, and their retail prices are really good.