Many of the retail sporting goods stores will have refill stations for your cylinders. Also, I have seen hardware stores that will do that too.
Where did you buy the gun? Chances are that the store the gun was purchased from will be able to refill the cylinder for you.
At any paintball field, and some sporting goods stores or gas shops (ones that sell tanks of propane and oxygen).
Sporting goods stores like Dick's, and Some "gas" stores that sell propane and oxygen tanks.
At some sporting goods stores or any paintball field.
At most Sporting goods, gun, and paintball fields. Some Gas (not just gasoline, but Ike propane) places you can fill your CO2.
At any paintball field, and most sporting goods stores. If there are none near you , just search online.
At some sporting goods stores, and all paintball fields.
At any local paintball store.
Take it to a shop! if you want to do it yourself the normal way is to put the bottle on scales and fill it from a larger bottle to a fill weight. To be honest though CO2 is well out of date. A smart move would be to move onto compressed air.
No paintball gun will work without a CO2 tank or other propellant tank. without Co2 there is nothing to push the ball out of the barrel.
if your tank runs out take it to your locale paintball field thay should fill it up for you for a small fee exp=5$ unlimated air for the day at my field.
Where the air tank or co2 tank normally attaches
There is no such thing as a "CA" tank.
A paintball gun, unlike airsoft or real firearms will have a hopper on top, and will have a co2 tank sticking out of the back.