[Vendan]

It's a bit expensive, but you can get adapter kits that let you hook a sodastream machine up to a standard CGA 320 CO2 tank, and then you can hook up a 5 or 10 pound CO2 tank. You'd probably be looking at $10~30 for a refill on one of those, which is GREAT considering the standard sodastream tank is only a 1 pounder.

Edit: also, forgot to mention. While your average sports shop paintball gun CO2 refill is dirty as crap, it's really common to have someplace around that has beverage quality CO2 (check breweries, they usually capture the excess from the brewing), and surprise surprise even though the beverage companies make a big fuss about their CO2, your average industrial CO2 supplier is giving you the same CO2 you'd get for food grade. Also look for kegerator suppliers, as they'd be another place to get CO2.

[toomanybeersies]

When I used to have a keg setup, I just filled it at the industrial CO2 supplier.

I never had any issues with dirty CO2, neither did the dozen or so other people I know who also kegged. Carbon dioxide is carbon dioxide as far as I'm concerned.

[Vendan]

Absolutely. I can't find the source, but I remember reading somewhere that the beverage companies put together a "CO2 Purity Requirements" thing, and it turned out industrial CO2 was already way more pure. Surprise!

[gtvwill]

The purity in gasses mostly refers to the presence of water vapour or machine oils. A great example of machine oil presence is in cheap nitrous canisters. Easy to see, just go find someone hitting the hippie crack at a party, wait till they finish a hit and ask to have a geez at their Bulbinator. Unscrew that bad boy and wipe your finger around the inside of the canister. Will 9 out of 10 times come out with oil residue on it. Nasty shit.