Yep there are loads of extinguisher service & refill companies. Smaller ones you'll wait a week or get an exchange unit as they can't fill them and have to send them off to a trade filling station, the larger ones with their own CO2 transfer pumps will fill on their site and if it's only a couple of extinguishers wil give a quicker turnaround (while you wait, same day, next day).
If your CO2 extinguishers have had 10 years pass since the last mm/yy date stamped in the neck then they will require an overhaul incorporating a cylinder safety pressure test. In which case you will be waiting at least a week if you insist on getting the original extinguishers back or otherwise you do a straight swap with your existing extinguisher for one that has been tested, , revalved, resprayed & refilled.
Some companies will insist on trying to sell you a new extinguisher if yours is >10 yrs but this is an unecessary expense (a new one only lasts 10 years at a time between tests anyway)