Related info: you can go to any local business and check their Fire Extingusihers for the hang-tag on their bottles to get the name of a local Fire Extinguisher Service company. Some are small independents willing to help a guy out, others more corporate, unwilling to bend any rules.
On the welding tanks it really pays to shop around on CO2 refills. Some of the statewide and nationwide chains are real scalpers. One trick they'll pull is when you go to exchange tanks they check their stock and always give you the one that's closest to expiring, maybe in just a month or two. That way they know they'll sting you for a hydrote$t next time you're in! I much prefer small locals to large chain operations.