I came across this thread because my Co2 tank is empty and I couldn't find anybody that would flat-out fill my tank and let me keep it so I wouldn't have to get rid of my new tank.
Anyways, the last guy posted that Koorsen Fire and Security fills tanks. Indeed they do, as I'm getting ready to head there myself. Honestly, I don't know if I'm going to get charged anything because I lucked out and know one of their employees. He actually took a 5lb fire extinguisher and set it up so I could use it to push my kegs, filled it and gave it to me for free.
So, I called him this morning to ask if they would fill it if I brought it in myself and how much they charge. He said he would call over there and tell them I'm heading in to get a tank filled for him and they wouldn't charge me. Plus, he's going to get me a spare for no charge. Sometimes it really is all about who you know!