Not that kind of gas! Since I had a little run around, I thought I would share and hopefully help someone in the future if they do a search.
First thing first, best/cheapest place in town to get CO2 (and your tank hydro tested) is at Madison Recharging Service. They do fire extinguishers, safety equipment, etc. A 20# Co2 fill is only $11.50 plus tax, and they do it right away. By comparison, Badger Welding charges $12.50, but only if you leave the tank. It costs an extra $10 to have it filled on the spot.
If your looking for beer gas, and go with Badger welding, DO NOT buy your own tank! Luckily I called there first and talked to a couple of different people on different days (seeing if I always got the same answer....I did). They basically can "try" to do it for you, but cant guarantee a correct mix. Instead you can either buy or rent a 55CF tank. If you buy it, its $130, and fills are $15.46. You dont buy a specific tank, you just buy a tank that gets fills. That way you can just walk in and exchange it for a new one, and you dont have to pay for hydro testing and the like. You can also rent one, but if you keep it for more than 2 years, you are better off buying said tank.
There are a couple of other places in town, but they were way out of my way (I live in Cross Plains and most were on the East Side). The only place I can say for sure to stay away from is Jefferson Fire supply in Middleton. They want INSANE amounts of money for hydro's and fills. Like 3x the price of everywhere else.
Hopefully this helps someone else out.