Ok ... update.
There is a comprehensive CO2 testing document put out by the US General Services Administration
here. In a nutshell there is medical grade and industrial grade. Medical is the purest at a much higher cost than the industrial grade.
I found a local place (CamCarb) that rents CO2 tanks to local businesses and private individuals. They want $50.00 to fill it up. They also have a program where they rent you a 20lb tank for $50.00 a year. If you can drain it 10 times a year this may not be a bad option. On the other hand if you have your own 20lb tank it would cost you $50.00 to fill it. A tad on the expensive side. The other benefit of this exchange program is that they own it and are responsible for the maintenance on the tank.
Big Al's Aquarium will only fill a tank if it was purchased from them. Cost of 20lb tank was $239.00. No thank you.
Paint Ball Nation had a policy of $6.50 plus tax per fill, yes, even a 20lb tank. I like this price.
Best price I found on a brand new 20lb tank is from the Beverage Factory.
(Click on tank for link.)It is $126.90 shipped from California to Niagara Falls, New York.
There are some other interesting links I've found that make a lot of sense in terms of setting up your own operation using disposable PET bottles with a little elbow grease and cheap supplies.
I'll see if I can dig them up later and post them.