I would also sugest
Buying it from a reputable local source if new and then see if they will fill it for you.
Or making friends with your local site owner we don't all bite.
if you are looking for a constant supply then I agree with all of the above but make sure you are doing it safley Co2 can give you a nasty freeze burn at the best and if the bottle is not securly attached it can or you are not holding it can really wind you god help you if it hits the jewlz
Remember with co2 you should always wear gloves and eye protection and never hold the bottle to your ear. I assume you are talking new equipment but if it's second hand check the marker bottle is with in date is sound so no bumps or dents.
If you are planningto shot yur marker at home take a few sencible precortions netting and warning the neibour if you have any always better to have them on side than the police round every 2 mins.
sparklie