It's not in the city, but I just found a fire extinguisher store out in Marlboro, just off route 20 downtown. I bought a 10lbs, black painted canister for $80 filled.
If I remember correctly, they will fill any tank 15 lbs or less for $15, and larger tanks for $25.
Not too bad of prices if you ask me!
Strange Brew just down the street has 5 pounders for $80, and dual gauge Cornelius regulators for $65.