actually not true. Someone has made a refillable co2 powerlett. How ever he had to fit a regulator on the top and it would not fit into his mags with out modifying it to stick out the bottom. I looked into this when I had my co2 GBBR. Basically you are stuck buying them. Cheapest I found was .78c at canadian tire if you buy 40.
Another alt method is to modify the bottom of one with a quick release out of the bottom and run a remote line, mag changes will involve you quick releasing and adapting to the next one, basically the powerlett becomes part of the mag.
There yah go, he also sells them for $20 each.
Looking at his new ones that actually looks like they would fit well.. hmmm