where to top up CO2 gas?

Hi, need some help from you guys. I'm now looking for a bigger CO2 cylinder because my 1/1.5l is too small. I know Nature Aquarium offers a one to one exchange but what exactly are the ones they exchange for? Found a few second hand cylinders on sale at src... dunno which one can be exchanged at NA when I need to top up.

http://www.sgreefclub.com/forum/inde...c=44081&hl=co2

http://www.sgreefclub.com/forum/inde...c=43497&hl=co2

http://www.sgreefclub.com/forum/inde...c=43557&hl=co2

Are these the ones that could be exchanged at NA? Thanks for the advice.

Such kinda questions seems not directed at fish keepers.
You will have better luck if you go down to the shops and ask the
aquarium shop owners which provide CO2 refill service.