extinguishers are commonly refilled and repressurized.
There is not much too one really although I am sure there is some disclaimer needed here in our legalese society.
Anyway, all that an extinguisher is is a bottle, holding powder, and nitrogen normally as the propellant, a valve on top which also usually serves as a handle and a nozzle to aim it up.
What you want is a good quality extinguisher with a metal head. You can service them but honestly it is not that expensive to have one serviced by the local shop and they have the adapters etc. ready to go.
Short version is that if you decide to do it yourself, carefully empty, weigh the contents, insure you have proper amount of contents, inspect cylinder carefully (hydrostat dates etc.) refill with powder then take off the nozzle, screw in an adapter from a regulated nitrogen source and refill, take adapter off and put the nozzle back and away you go.
Again though, unless you have the lines, adapters, nitrogen, regulator, scales etc it is a lot more straight forward to go to the local FE shop and have them do it, generally it is pretty cheap.
They can also be a good source of used (cheaper) extinguishers if they choose and I noticed that my local Habitat for Humanity RE-Store (selling used building items) had a large assortment there.
Good luck, oh, and PS, the charge has to be dry, normal shop air would have too much moisture and the powder would be a brick when you wanted to use it.