refill dry chemical extingishers

Standard stored-pressure extinguishers can be refilled too, if you know what you are doing. Ext. agent can bought in bulk, but you would need a source of dry air or nitrogen for the pressure charge. Moist air will cause the agent to cake.
I doubt that you can do it any cheaper than an extinguisher servicing company can do it for you though.

After I got out of the army in 1970 I got a job for a fire protection equipment company in Honolulu. We refilled dry chemical extinguishers all the time.

The commercial chemical in small extinguishers was baking soda with an anti-caking agent. Annual service of an ABC rated unit was to turn it upside down and hit the bottom with a rubber mallet, then shake it to make sure you could feel the chem. move. Then you flick the gauge with you fingertip to see the needle quiver to make sure it isn't stuck.

All the best, Stan