My suggestion is to find a speed shop, tell them what you are doing. Most will sell you tanks, or will install a valve in a CO2 tank to convert it. My tanks were CO2 tanks that had the valves swapped out by the speed shop. Remember to make sure you have a siphon tube. You want to draw liquid not gas from the tank.
Be careful of used tanks that are out of hydro, you will need to add the cost of the hydro, cheep tank may not be so cheep after that. If you do buy a new CO2 tank, make sure that It was recently tested, not fun to buy a new tank and realize it was a few years old, not a performance problem just how long before hydro testing.
A brand new 20lb CO2 tank WITH A HANDLE should be less than $150 delivered. The handle, many of the tanks for cars don't have a handle, trust me, get one with a handle. It also protects the valve when transporting it.
One last thing, years ago I started adding a short (2 foot) hose to the end of my fill hose (the braided steel hose attached to the regulator, not the plastic hose coming from the motor). The end of the hose can get damaged from the exhaust from the motor or from being whipped. Cheep to replace the 2 foot piece, and to keep a spare in case it gets damaged during a day of flying. One last thing, buy a cap for the valve outlet for when you transport the tank.