Answers:
Carbon Dioxide or CO2 is readily available from Williams Sonoma stores surrounded by cartridges. You can probably mail demand it if you call them though it's not sold on their website.
They use it contained by their seltzer siphons and whipped cream pumps.
Other Answers:
most companies that service restraunt's soda dispensers hold it in stock within 20 pound containers {steel bottles} it is expensive!
sure you can buy it from any local coke or pepsi distributor, but you will have to pay envelope a deposit on the tank
you can probably find some at a oxygen supply company[medical supplier] or at a weld supply house
probably any place that sells close to oxygen tanks and weld tanks
own a great day
you can get carbon dioxide by exhaling
If you want it in a reservoir, go online