You can refill a paintball tank from a big tank pretty easily. The key is to remember that CO2 is stored as a liquid, not a gas.
BTW, smallest paintball tank I ever found was a 3.5oz. 9oz is pretty common.
To refill a paintball tank, you'll need a contraption that's roughly like the following:
A tee connector. One leg goes to the big CO2 tank. One leg goes to the paintball tank. One leg has a valve and exhausts. See
https://www.google.com/search?q=co2+paintball+filling+station
1: attach the big and small tanks. Close all valves, including the big tank's screw valve. Paintball tanks normally just have a pin valve.
2: Open the valve to the paintball tank.
3: Open the valve to the big tank.
4: Let pressure equalize.
5: Close the valve to the big tank.
6: Open the dump valve.
-- at this point the paintball tank should be cold.
7: close the exhaust valve
8: Turn the big tank upside-down and hold it above the paintball tank.
9: open the valve on the big tank and fill the little tank.
10: close paintball tank valve
11: close big tank valve
12: open exhaust valve to equalize the system
The goal is to chill the paintball tank (so CO2 stays liquid) and then fill it with liquid - hence the upside-down.