A sodastream bottle, or more accurately Gas Cylinder is a 330g cylinder used in home soda making machines called Sodastreams. I actually thought that they were a reasonably international phenomenon.
A fair few people have adapters made for them and use them with regulators, as C02 supply in party keg systems. I took a more direct route and just butchered the whole soft drink machine and use it to power my homemade party keg system (which untill VERY recently was my only keg system) The photo below shows the Drink machine with my added Gas line and schrader valve filler, on the right is the party keg made froma garden pressure sprayer, schrader gas in valve and a picnic tap. It all works a treat and will hold enough pressure to force carb as well as serve. Handy little setup.
Normally the cylinders cost about $12 to swap for a full one, and about $45 to get without swapping. Safeway was a bit slack and I picked up a couple of keepers for the swap price.