Halon is in limited supply and quite expensive now. Any firefighter's supply and yellow pages, if they still exist or the 'net are your friends here, can refill extinguishers. A rule of thumb given me a few years ago by my local supplier - if your extinguisher has a plastic nozzle, it wont be refilled (note I said wont not can't). Metal nozzle, yes, no problem (normally). With that said, and unless it is a big extinguisher 5# or more, it is less expensive to purchase new as the old may need the added expense of a fresh hydro test due age.
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')