You can get it from welding supply stores, Praxair, BOC, Airgas and a bunch of other distributors and producers have their own stores.
If available try getting beverage or USP grade whose purity is better controlled than the welding / industrial or extinguisher grades.
Some safety tips:
The CO2 in the cylinder is stored at 800 to 1000 psi of pressure, if the valve or regulator breaks that is enough pressure to send the cylinder flying like a rocket and at least destroy everything under your stand and make a couple of nice holes through the wall potentially causing serious injuries, so secure the cylinder (chain or rope it to a wall) when in use to prevent it from flipping over. Always use the cylinder standing up other wise liquid may be withrawn and cause freezing and failure of the regulator.
The CO2 is not toxic but because heavier than air, if released may displace the air and cause asphyxia. Insure there are no leaks in the installation and insure the area is well ventilated while making adjustments or tests that require releasing some CO2.
While transporting it, never leave a cylinder unattended in a parked car. When going for a refill make picking up the CO2 the last chore of the trip. Keep the cylinder in a cool shady place.
When filling a cylinder it becomes very cold (Below zero) the liquefied gas inside is condensed due to the temperature. When the temperature of the liquid rises to ambient the liquid expands and if the cylinder is overfilled (usually when not using a scale) the liquid may not actually fit inside the cylinder thus increasing the pressure to the point of blowing the safety. This is a very dangerous situation as all the gas is released at once so insure the cylinder is filled using scales (Many extinguisher places do not do so) and insure the cylinder is at ambient temperature when you receive it.
To prevent damage to the regulator gauges, before opening the cylinder valve always unscrew the pressure adjusting knob in the regulator, then slowly fully open the cylinder valve, check for leaks and then screw in the pressure adjusting knob until the output gauge reads your use pressure (usually between 15 to 25 psi).
Always use a check valve between the regulator and the reactor to prevent salt water back flowing into the regulator.
Enjoy and be safe!
