I don't want to rain on what is in theory a good idea but I would caution you against this if you have a choice.
I posted about this somewhere else but can't link
The main points:
CO2 is not CO2 is not CO2. It comes in all sorts of grades, you should get food grade, not fire extinguisher grade. The grade is way more important than purity.
Even if it is 99.9 or even 99.99% it's still no good if it's not food grade and 99%...forget it. That 0.1 or 0.01% could contaminated with really nasty crud. For the sake of a few bucks and your well being I'd urge you to insist on food grade and check it is.
It's up to you, here is food grade spec
http://www.logichemprocess.com/CO2 Food Grade Specs.pdf
These limits are deemed safe, to put a fire out it doesn't matter
Secondly there is the containers that can cross contaminate if corroded etc.
Sure this is a way to rip you off, and 99999/100,000 it won't matter, like insurance I guess, you don't need it til you need it.
Sometimes perhaps mostly, ordinary CO2 might meet this spec, sometimes not.
Look at this
http://www.jmcatalysts.com/ptd/site.asp?siteid=671&pageid=674
there is a reason why benzene is set lower for consumption than for processing.
like this for example
http://en.wikipedia.org/wiki/Benzene_in_soft_drinks
It all depends how you value your health. But paintball-fire-extinguisher-bike tire refills etc would be best avoided unless certified at food grade and certified means US or European not eBay or china. Sure you can save a ton of money by buying things unfit for human consumption.....
If you want cheap and easy bottle carbonate in cola bottles.