Last time I checked, ebay had 20 oz. bottles for $17, which was the cheapest I could find. I don't know much about paintball bottles, but I'm assuming that there shouldn't be much difference between one brand or another.
I would opt for new ones if I were you, you never know what someone's done with a used one.
I've bought mine on ebay, amazon, and at Academy sporting goods. I would recommend buying at least two, so you can immediately swap out when you're empty. I've got paintball CO2 on 3 aquariums so I have 6 tanks.